Hi, this is my first post here but I've been reading this forum for some time and I'm learning by the day.
I have a pair of mono valve amplifier chassis from old pye black box record players (bought cheaply off ebay), they each use a single ECC83 for the input stage, two EL42's for the output stage and one EZ41 rectifier.
Unfortunately they aren't a matching pair, one is the early mk 1 circuit the second being the mk 2 which differs in that it uses 'an ultra linear push-pull output stage'. so the output transformers are different, one has three wires for an input the other has five.
I have tested both amplifiers and they both work, they have an output of about 2 watts each I would estimate.
now I honestly don't know much about valve amplifiers and I would like to know if these amps would be a good basis to build a stereo amplifier or if these tubes are too lo-fi for any practical use? does anyone have a schematic for an amplifier using these valves? I have the schematic for the black box but I assume the original design, being designed for a crystal stylus and not a CD player would need serious modification which i'm afraid I don't know how to do.
any advice/thoughts or schematics using these valves would be greatly appreciated
Phil.
I have a pair of mono valve amplifier chassis from old pye black box record players (bought cheaply off ebay), they each use a single ECC83 for the input stage, two EL42's for the output stage and one EZ41 rectifier.
Unfortunately they aren't a matching pair, one is the early mk 1 circuit the second being the mk 2 which differs in that it uses 'an ultra linear push-pull output stage'. so the output transformers are different, one has three wires for an input the other has five.
I have tested both amplifiers and they both work, they have an output of about 2 watts each I would estimate.
now I honestly don't know much about valve amplifiers and I would like to know if these amps would be a good basis to build a stereo amplifier or if these tubes are too lo-fi for any practical use? does anyone have a schematic for an amplifier using these valves? I have the schematic for the black box but I assume the original design, being designed for a crystal stylus and not a CD player would need serious modification which i'm afraid I don't know how to do.
any advice/thoughts or schematics using these valves would be greatly appreciated
Phil.
What makes you wanna to go for EL42's ? Awkward 8 pin sockets. 2W is a crumb low. A push pull pair should get 7W with 250V B+ on anodes other than this, in my 1950's audiobook (with cover missing) mentions "quality audio amp".
I strongly suspect one model has the o/p stage with a common cathode resistor (unbypassed) with reasonably matched tubes. Or using o/p transformers with differing inductances.
I suspect the anode to anode impedance is around 15 K....bit high implying a ratio of 70:1 for a 3-4 Ohm LS
As Franks data sheet mentions....this tube may have seen it's debut in early day car radios with vibrators.
I will dig the circuit from my archives.
richj
I strongly suspect one model has the o/p stage with a common cathode resistor (unbypassed) with reasonably matched tubes. Or using o/p transformers with differing inductances.
I suspect the anode to anode impedance is around 15 K....bit high implying a ratio of 70:1 for a 3-4 Ohm LS
As Franks data sheet mentions....this tube may have seen it's debut in early day car radios with vibrators.
I will dig the circuit from my archives.
richj
Phil, please post a scan of the schematic. Little change is required to switch from crystal phono cart. as the source to CDP as the source.
Ultralinear (UL) topology involves connecting the screen grids of pentodes to taps on the O/P trafo primary winding, instead of directly to a high positive voltage. The British description of partial triode is very apt. UL mode is, in fact, a form of local NFB.
Obviously, assuming the 2 O/P trafos are otherwise equivalent, you will have to use pentode or triode mode in both channels and tie the UL tap wires off.
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As Rich has indicated, things start with the EL42 data sheet. The rim lock socket is a PITA. Fortunately, as shown here, the type is electrically equivalent to the Noval based EL85. The EL85 data sheet contains a good deal of useful info. that confirms Mr. Walter's excellent intuition.
Phil,
The classic telefunken V73 uses EL42 tubes. This push pull amp has a cult following in Europe and commands very high prices. Even some US tubeophiles appear to think highly of this amp.
http://www.oswaldsmillaudio.com/2003a.html
In their day, Pye tended to dominate the opposite end of the budget spectrum, so I doubt whether the EL42s would be the limiting factor in their amp's performance.
The classic telefunken V73 uses EL42 tubes. This push pull amp has a cult following in Europe and commands very high prices. Even some US tubeophiles appear to think highly of this amp.
http://www.oswaldsmillaudio.com/2003a.html
In their day, Pye tended to dominate the opposite end of the budget spectrum, so I doubt whether the EL42s would be the limiting factor in their amp's performance.
Thanks for the help guys, much appreciated.
The reason I'm trying to use these valves and transformers is simply because I have them and I'm hoping to build an amplifier on a low budget. 7w per channel is also more than I hoped with these little tubes
as for the tube sockets I'm hoping to reuse the sockets I already have if I can.
I will try and post the schematic I have on here tonight, unfortunately I have to head off to work now..
Thanks again for your responses guys.
Phil.
The reason I'm trying to use these valves and transformers is simply because I have them and I'm hoping to build an amplifier on a low budget. 7w per channel is also more than I hoped with these little tubes
as for the tube sockets I'm hoping to reuse the sockets I already have if I can.
I will try and post the schematic I have on here tonight, unfortunately I have to head off to work now..
Thanks again for your responses guys.
Phil.
Hi Again, I've opened a photobucket account so that I can post clear images of the tatty service sheet I have, hopefully these images will work...
The schematic is for the Mk2 version, on page two there is a discription describing the differences between this and the earlier model.
Eli Duttman thanks for letting me know that the modifications are possible. It would be fantastic if you could modify the schematic for me so that it will be possible to connect a CD player directly to it and also, if possible remove the tone control as I would prefer not to have this in the circuit. I would be very greatful, please have a look and see what you think.
I am thinking that it will be easier to downgrade the later model than upgrade the earlier model as I think I will be able to find a 'three terminal input' output transormer more easily than another 5 terminal one, is this a sensible idea? or is loosing the ultra-linear feature a bad idea? obviously I need a matching pair for a stereo amp.
also thanks mach1 for the link to the V73, I had no idea there was so much potential in the EL42's!
thanks again, Phil.
The schematic is for the Mk2 version, on page two there is a discription describing the differences between this and the earlier model.
Eli Duttman thanks for letting me know that the modifications are possible. It would be fantastic if you could modify the schematic for me so that it will be possible to connect a CD player directly to it and also, if possible remove the tone control as I would prefer not to have this in the circuit. I would be very greatful, please have a look and see what you think.
I am thinking that it will be easier to downgrade the later model than upgrade the earlier model as I think I will be able to find a 'three terminal input' output transormer more easily than another 5 terminal one, is this a sensible idea? or is loosing the ultra-linear feature a bad idea? obviously I need a matching pair for a stereo amp.
also thanks mach1 for the link to the V73, I had no idea there was so much potential in the EL42's!
thanks again, Phil.
Phil,
You are definitely on the right track by wanting to eliminate the tone controls, as they are very nasty.
As is so often the case with "ancient" units, considerable simplification is a must. Earlier, you mentioned that you were not looking for the peak power possible from PP EL42s. Triode wiring the "finals" in both amps allows you to use the O/P trafos already in your possesion. Just tie the UL taps off on the part that has 'em. Figure on an approx. 50% reduction in peak power O/P, but all sorts of pitfalls disappear, when that price is paid. To triode wire, connect the screen grids to the plates via 100 Ohm resistors.
Are you willing to go back and forth between the 2 units when setting listening level and channel to channel balance? If so, a simple log. taper pot. at the I/P of each amp is all that's necessary.
For more than 1 reason, the small signal circuitry needs to be revised. Once the tone control stuff gets tossed, you have too much gain. The bandwidth a CDP provides is considerably broader than the BW a crystal phono cart. delivers. The posted schematic shows a common cathode gain block cap. coupled to a split load phase splitter. The splitter is cap. coupled to the PP "finals". That many coupling caps. in an amp with global NFB and low quality O/P trafos is a prescription for instability. Aside from excessive gain, an ECC83/12AX7 section has a high RP and low gm that make it a poor splitter, especially for driving the substantial Miller capacitance of triode wired O/P tubes. Fisher, Dyna, et al, DC coupled the splitter to the voltage amplifier, which makes a world of difference in the stability dept. Perhaps some other twin triode could be used with the existing B+ PSU in a DC coupled arrangement.
Do you happen to know how much current the pilot lamp draws? If it's 150 mA. or more, that current could be used to increase the possibilities for the driver/splitter position.
You are definitely on the right track by wanting to eliminate the tone controls, as they are very nasty.
As is so often the case with "ancient" units, considerable simplification is a must. Earlier, you mentioned that you were not looking for the peak power possible from PP EL42s. Triode wiring the "finals" in both amps allows you to use the O/P trafos already in your possesion. Just tie the UL taps off on the part that has 'em.
Figure on an approx. 50% reduction in peak power O/P, but all sorts of pitfalls disappear, when that price is paid. To triode wire, connect the screen grids to the plates via 100 Ohm resistors.Are you willing to go back and forth between the 2 units when setting listening level and channel to channel balance? If so, a simple log. taper pot. at the I/P of each amp is all that's necessary.
For more than 1 reason, the small signal circuitry needs to be revised. Once the tone control stuff gets tossed, you have too much gain. The bandwidth a CDP provides is considerably broader than the BW a crystal phono cart. delivers. The posted schematic shows a common cathode gain block cap. coupled to a split load phase splitter. The splitter is cap. coupled to the PP "finals". That many coupling caps. in an amp with global NFB and low quality O/P trafos is a prescription for instability.
Aside from excessive gain, an ECC83/12AX7 section has a high RP and low gm that make it a poor splitter, especially for driving the substantial Miller capacitance of triode wired O/P tubes. Fisher, Dyna, et al, DC coupled the splitter to the voltage amplifier, which makes a world of difference in the stability dept. Perhaps some other twin triode could be used with the existing B+ PSU in a DC coupled arrangement.Do you happen to know how much current the pilot lamp draws? If it's 150 mA. or more, that current could be used to increase the possibilities for the driver/splitter position.
Thanks Eli, I think I follow you on the tone control removal, would it be ok to replace pot R18 for a resistor of the same value, removing the wire that connected the 'wiper' of R18 to C2 and removing C2, is this all thats required?
regarding the volume control am I right in thinking I can connect a log pot (say 50k?) directly to the grid of V1a and earth and I can do away with R1, R2, R3, C1 and R4? Sorry if this is a dumb question, also, do i need a decoupling cap? if so where should it go?
now you said "To triode wire, connect the screen grids to the plates via 100 Ohm resistors" I'm afraid I'm not used to this terminology, by plate do you mean the Annode?
now I must admit you lost me a little with the small signal circitry revision, I would prefer to use the tubes I have, they are all original Mullard and also I would like to keep the cost of this project to a minimum, but if thats unwise then I'm happy to change them for something better.
I haven't seen the lamps used in the black boxes, but I would assume its just a small flashlight style bulb, about 150mA seems a fair estimate.
thanks for your time, I hope you don't mind all these questions.
Phil.
regarding the volume control am I right in thinking I can connect a log pot (say 50k?) directly to the grid of V1a and earth and I can do away with R1, R2, R3, C1 and R4? Sorry if this is a dumb question, also, do i need a decoupling cap? if so where should it go?
now you said "To triode wire, connect the screen grids to the plates via 100 Ohm resistors" I'm afraid I'm not used to this terminology, by plate do you mean the Annode?
now I must admit you lost me a little with the small signal circitry revision, I would prefer to use the tubes I have, they are all original Mullard and also I would like to keep the cost of this project to a minimum, but if thats unwise then I'm happy to change them for something better.
I haven't seen the lamps used in the black boxes, but I would assume its just a small flashlight style bulb, about 150mA seems a fair estimate.
thanks for your time, I hope you don't mind all these questions.
Phil.
Phil,
Yes, plate = anode. Please use the 100 Ohm triode mode "straps".
Mullard made ECC83s from the 1950s are very good tubes. Unfortunately, they are electrically wrong for the task at hand. Store the ECC83s very carefully. You will use 'em in a future phono preamp project.
"Stealing" the current used by the on/off indicator lamp allows the use of a 12AV7 family type. Other members of the family are the 5965, the 7062/E180CC, and the CV10175. Gain is still substantial, but low RP and high gm make for a much better driver/splitter.
I'm hoping to use DC coupling between small signal stages. Eliminating the coupling cap. there is important for stability's sake.
Please use "Google" to research Miller capacitance. That phenomenon is yet another issue with the 12AX7/ECC83.
High gm triodes are vulnerable to parasitic oscillation. You deal with the issue by mounting 1 KOhm Carbon composition resistors right at the grid socket lugs. The "free" end of a stopper resistor is where the grid connection is made.
A 50 KOhm log. taper volume control is OK. Couple the wiper of the volume control via a 12 nF. cap. to the grid stopper. Connect the grid stopper to ground via a 510 KOhm resistor. A high pass filter that "corners" at approx. 26 Hz. is formed. That filter provides important protection against core saturation in the O/P trafo. All the OEM parts in the I/P grid circuit get discarded.
R18 and C2 get discarded. R17 becomes a 1% tolerance 1 KOhm metal film part and connects to the junction of R5 and R6, not the cathode of V1a. R6 becomes a 1% tolerance 100 Ohm metal film part. The new value of R5 is to be determined (TBD). That cathode bias part has to be bypassed by a large electrolytic capacitor.
C3 is replaced by a KOhm Carbon comp. grid stopper. R8 and R10 are discarded. The junction of R9 and C6 is at the cathode of V1b. R9 and R11 are of an identical value, which is TBD. I'm hoping that SY will speak up in this thread, as he can work DC coupled "concertina" phase splitters out, "on autopilot".
The thought of 50 Hz. heater power flowing in the chassis makes me VIOLENTLY ILL. Tightly twisted pair wiring must be used for AC heater power. Lift that 6.3 VAC connection to ground!
Yes, plate = anode. Please use the 100 Ohm triode mode "straps".
Mullard made ECC83s from the 1950s are very good tubes. Unfortunately, they are electrically wrong for the task at hand. Store the ECC83s very carefully. You will use 'em in a future phono preamp project.
"Stealing" the current used by the on/off indicator lamp allows the use of a 12AV7 family type. Other members of the family are the 5965, the 7062/E180CC, and the CV10175. Gain is still substantial, but low RP and high gm make for a much better driver/splitter.
I'm hoping to use DC coupling between small signal stages. Eliminating the coupling cap. there is important for stability's sake.
Please use "Google" to research Miller capacitance. That phenomenon is yet another issue with the 12AX7/ECC83.
High gm triodes are vulnerable to parasitic oscillation. You deal with the issue by mounting 1 KOhm Carbon composition resistors right at the grid socket lugs. The "free" end of a stopper resistor is where the grid connection is made.
A 50 KOhm log. taper volume control is OK. Couple the wiper of the volume control via a 12 nF. cap. to the grid stopper. Connect the grid stopper to ground via a 510 KOhm resistor. A high pass filter that "corners" at approx. 26 Hz. is formed. That filter provides important protection against core saturation in the O/P trafo. All the OEM parts in the I/P grid circuit get discarded.
R18 and C2 get discarded. R17 becomes a 1% tolerance 1 KOhm metal film part and connects to the junction of R5 and R6, not the cathode of V1a. R6 becomes a 1% tolerance 100 Ohm metal film part. The new value of R5 is to be determined (TBD). That cathode bias part has to be bypassed by a large electrolytic capacitor.
C3 is replaced by a KOhm Carbon comp. grid stopper. R8 and R10 are discarded. The junction of R9 and C6 is at the cathode of V1b. R9 and R11 are of an identical value, which is TBD. I'm hoping that SY will speak up in this thread, as he can work DC coupled "concertina" phase splitters out, "on autopilot".
The thought of 50 Hz. heater power flowing in the chassis makes me VIOLENTLY ILL. Tightly twisted pair wiring must be used for AC heater power. Lift that 6.3 VAC connection to ground!
Thanks again Eli for walking me through this, I really appreciate your time and effort and knowlege. This amp must be nearly there now, I'm really pleased with how its progressing. I'm learning a lot too, I read up on Miller capacitance, interesting stuff, the big revelation for me was, while reading about things you've mentioned, finding out about the different modes you can build your amplifier to work in (triode, pentode, UL, SE etc)and the pros/cons of each, this is a big leap forward for me and my understanding.
Right, back to business, I followed your instructions and modified my schematic during my lunch break today, here's what it looks like now:
is this ok so far? I think the only change you mentioned that I didn't put in was "That cathode bias part has to be bypassed by a large electrolytic capacitor." I just wanted to check, do you mean in parallel with R5? what kinda value is 'large'?
now the replacement driver tube, I've had a look and there are lots to choose from, different brands, different measured values for tested tubes, even different sizes! I was about to get these but I thought no harm in checking first, are these up to the job?
http://cgi.ebay.co.uk/ws/eBayISAPI....m=110254506808&ssPageName=STRK:MEWA:IT&ih=001
thanks again, Phil.
Right, back to business, I followed your instructions and modified my schematic during my lunch break today, here's what it looks like now:
is this ok so far? I think the only change you mentioned that I didn't put in was "That cathode bias part has to be bypassed by a large electrolytic capacitor." I just wanted to check, do you mean in parallel with R5? what kinda value is 'large'?
now the replacement driver tube, I've had a look and there are lots to choose from, different brands, different measured values for tested tubes, even different sizes! I was about to get these but I thought no harm in checking first, are these up to the job?
http://cgi.ebay.co.uk/ws/eBayISAPI....m=110254506808&ssPageName=STRK:MEWA:IT&ih=001
thanks again, Phil.
Phil,
If you are going to buy new driver tubes in the US, I suggest you purchase from Jim McShane, as he is a dealer with a sterling reputation. For not much extra, you can have black plate RCA 12AV7s, which will be quite yummy in combination with the Mullard made EL42s. Look closer to home for Dutch Philips E180CC, which are fine, but importing RCA from the US might cost less. Perhaps you can find CV10175s "locally". That is a British designation and could be the best deal.
The schematic is shaping up nicely.
Yes, R5 (whatever its new value turns out to be) gets bypassed. The "pole" frequency for that combo must be no greater than 5 Hz.
When the new value for R7 is determined, use an inductive wirewound part. Connect a 120 pF. mica or NPO ceramic cap. from the "hot" speaker terminal to ground. Between HF peaking (due to the inductance of R7) and the shorting of the NFB loop out above 80 KHz. (by the 120 pF. cap.) the NFB loop gets reasonably well phase compensated.
R12 and R13 should be low noise 470 KOhm metal film parts. C5 and C6 should be 100 nF./400 WVDC film and foil. 716P series "orange drops" are up to snuff for C5/C6.
Notice that R15 is not replicated at the control grid of V2. It should be. Use 10 KOhm Carbon comp. parts for R15 and R15 prime. Carbon comp. resistors are used as control grid stoppers because they are both non-metallic and non-inductive. "Horses for courses."
Good technique does not use the chassis for signal grounding purposes. Either the bus or the "star" method should be employed. The signal ground gets connected to the chassis at single point.
Don't forget to install a proper, 3 wire, safety grounded, power cord.
If you are going to buy new driver tubes in the US, I suggest you purchase from Jim McShane, as he is a dealer with a sterling reputation. For not much extra, you can have black plate RCA 12AV7s, which will be quite yummy in combination with the Mullard made EL42s. Look closer to home for Dutch Philips E180CC, which are fine, but importing RCA from the US might cost less.
Perhaps you can find CV10175s "locally". That is a British designation and could be the best deal.The schematic is shaping up nicely.
Yes, R5 (whatever its new value turns out to be) gets bypassed. The "pole" frequency for that combo must be no greater than 5 Hz.
When the new value for R7 is determined, use an inductive wirewound part. Connect a 120 pF. mica or NPO ceramic cap. from the "hot" speaker terminal to ground. Between HF peaking (due to the inductance of R7) and the shorting of the NFB loop out above 80 KHz. (by the 120 pF. cap.) the NFB loop gets reasonably well phase compensated.
R12 and R13 should be low noise 470 KOhm metal film parts. C5 and C6 should be 100 nF./400 WVDC film and foil. 716P series "orange drops" are up to snuff for C5/C6.
Notice that R15 is not replicated at the control grid of V2. It should be. Use 10 KOhm Carbon comp. parts for R15 and R15 prime. Carbon comp. resistors are used as control grid stoppers because they are both non-metallic and non-inductive. "Horses for courses."
Good technique does not use the chassis for signal grounding purposes. Either the bus or the "star" method should be employed. The signal ground gets connected to the chassis at single point.
Don't forget to install a proper, 3 wire, safety grounded, power cord.
Ok, I've Added C2 across R5, C8 across the output and cleaned up the schematic a bit:
I've also drawn up a parts list to keep track of the recommended component types etc:
A couple of questions, for the resistors are 1W rating ok? and the original smoothing caps C4 and C7, can I increase them to say both 47uF? I figure bigger is better with smoothing caps and I have some 47uF 450Vs lying around.
I intend to wire it 'star' ground and it will have the correct 3 core cable and fuse, no problem.
thanks, phil.
I've also drawn up a parts list to keep track of the recommended component types etc:
A couple of questions, for the resistors are 1W rating ok? and the original smoothing caps C4 and C7, can I increase them to say both 47uF? I figure bigger is better with smoothing caps and I have some 47uF 450Vs lying around.
I intend to wire it 'star' ground and it will have the correct 3 core cable and fuse, no problem.
thanks, phil.
Phil,
The schematic has shaped up very nicely. However, C8 should be 1200 pF., not 120 pF. That's my error, sorry. 
The 1100 Ohm sum of R17 and R6 combine with C8 to form a low pass filter. After allowing for the inevitable stray wiring capacitance, that filter's "corner" freq. will still be above 80 KHz.
Unless otherwise indicated, all resistors should be 1/2 W. rated. Carbon resistors come in 2 "flavors": composition and film. Carbon film parts are slightly inductive, which makes them unsuitable for grid stopper service. OTOH, Carbon film parts will be dandy in the 100 Ohm triode "strap" positions. Metal film resistors are inherently low noise and normally come with a 1% tolerance.
Vacuum rectifiers, like the EZ41, are intolerant of large valued cap. I/P filters. They will destructively arc over at turn on. The EZ41 Data Sheet indicates which situations are safe. The 25 μF. OEM value for R7 seems well thought out. The closest modern standard value is 22 μF. and that's what you should (IMO) use. The way to improve smoothing and increase energy storage, in the PSU, is to insert a LC section of 1 to 2 H. and 1 of your 47 μF. parts after the initial filter capacitor. R16 and C4 decouple the small signal circuitry from the power O/P stuff. 15 μF. should be quite sufficient for C4.
The schematic has shaped up very nicely.
However, C8 should be 1200 pF., not 120 pF. That's my error, sorry. The 1100 Ohm sum of R17 and R6 combine with C8 to form a low pass filter. After allowing for the inevitable stray wiring capacitance, that filter's "corner" freq. will still be above 80 KHz. Unless otherwise indicated, all resistors should be 1/2 W. rated. Carbon resistors come in 2 "flavors": composition and film. Carbon film parts are slightly inductive, which makes them unsuitable for grid stopper service. OTOH, Carbon film parts will be dandy in the 100 Ohm triode "strap" positions. Metal film resistors are inherently low noise and normally come with a 1% tolerance.
Vacuum rectifiers, like the EZ41, are intolerant of large valued cap. I/P filters. They will destructively arc over at turn on.
The EZ41 Data Sheet indicates which situations are safe. The 25 μF. OEM value for R7 seems well thought out. The closest modern standard value is 22 μF. and that's what you should (IMO) use. The way to improve smoothing and increase energy storage, in the PSU, is to insert a LC section of 1 to 2 H. and 1 of your 47 μF. parts after the initial filter capacitor. R16 and C4 decouple the small signal circuitry from the power O/P stuff. 15 μF. should be quite sufficient for C4.Phil,
In my previous post, I said "25 μF. OEM value for R7 seems well thought out". R7 should read C7.
In the latest schematic iteration posted, you still show the EZ41's heater as being connected to the chassis. Along with the signal tube heaters, the rectifier's heater should be "lifted" and hooked up via a tightly twisted pair.
Should it prove necessary to ground the heater supply, a center tap will be fabricated from a pair of resistors or a trim pot. and the connection to ground made by a 'lytic. When this type of circuitry is employed, biasing the heater supply up to approx. 90 VDC, with a resistive voltage divider across the B+ rail, is "routine".
In my previous post, I said "25 μF. OEM value for R7 seems well thought out". R7 should read C7.
In the latest schematic iteration posted, you still show the EZ41's heater as being connected to the chassis. Along with the signal tube heaters, the rectifier's heater should be "lifted" and hooked up via a tightly twisted pair.
Should it prove necessary to ground the heater supply, a center tap will be fabricated from a pair of resistors or a trim pot. and the connection to ground made by a 'lytic. When this type of circuitry is employed, biasing the heater supply up to approx. 90 VDC, with a resistive voltage divider across the B+ rail, is "routine".
Oops yes I missed the rectifier tube heater, again it will be included on the next revision.
I've come across a bit of a problem, well a few actually... I Went through the amps yesterday, testing the output transformers to make sure they matched...
The first problem is that the output transforms are marked 1.5 Ohm on the secondaries, they must have driven two 3 Ohms speakers in parallel (interestingly they actually both measure 0.5 Ohms across the secondary terminals?) so its going to be awkward to find such low impedance speakers, I would probably have to make my own.
and the second problem is that one of the OTs is open circuit on one half of the primary. Strangely I still got 'music' when I last tested the amp a few weeks ago, so either I was listening to one half of the output or it has broken since testing...?
oh and there is a THIRD problem (which I already knew) with the OT's but that’s just cosmetic, one (the working one) has been dipped in pitch which is flaking off and looks very ugly, I was wondering what to do about this, Its not gonna look good on top of the chassis.
Either way I figure this is quite a big deal. The output transformers are one of the main reasons to start with an old chassis instead of from scratch...
So my question is how cheaply can I replace these OT's? they are 16k across the input and I would like ideally 8 Ohms across the output. Is something like this available? I know there is a lot of variation in quality in OT's but at the same time this was supposed to be a low budget 'project'.
another concern (which might be undue but I'd better mention it) is that the output tubes of both amps are almost completely silver, is this normal or a sign that the tubes are on their way out?
I've come across a bit of a problem, well a few actually... I Went through the amps yesterday, testing the output transformers to make sure they matched...
The first problem is that the output transforms are marked 1.5 Ohm on the secondaries, they must have driven two 3 Ohms speakers in parallel (interestingly they actually both measure 0.5 Ohms across the secondary terminals?) so its going to be awkward to find such low impedance speakers, I would probably have to make my own.
and the second problem is that one of the OTs is open circuit on one half of the primary. Strangely I still got 'music' when I last tested the amp a few weeks ago, so either I was listening to one half of the output or it has broken since testing...?
oh and there is a THIRD problem (which I already knew) with the OT's but that’s just cosmetic, one (the working one) has been dipped in pitch which is flaking off and looks very ugly, I was wondering what to do about this, Its not gonna look good on top of the chassis.
Either way I figure this is quite a big deal. The output transformers are one of the main reasons to start with an old chassis instead of from scratch...
So my question is how cheaply can I replace these OT's? they are 16k across the input and I would like ideally 8 Ohms across the output. Is something like this available? I know there is a lot of variation in quality in OT's but at the same time this was supposed to be a low budget 'project'.
another concern (which might be undue but I'd better mention it) is that the output tubes of both amps are almost completely silver, is this normal or a sign that the tubes are on their way out?
Phil,
O/P "iron" is a BIG deal. It's heavy. So, long distance shipping of otherwise modestly priced items can turn out to be no bargain, at all. "Local" suppliers frequently wind up costing you less, in toto.
The UK trafo winders I'm familiar with are Sowter and Stevens & Billington. Sowter's model U063 has the correct impedances, but its power handling capability is questionable. Brian Sowter is a very helpful fellow. Send him an EMail explaining your situation. Something similar to the U063 on the next larger size laminations is a distinct possibility. While you are at it, consider ultralinear taps.
I'd like to think that the circuitry under discussion is highly competent. So, investing in decent O/P trafos will pay off.
BTW, that 1.5 Ohm impedance you came up with makes sense. Paper cone speaker drivers with a 3.2 Ohm impedance were ubiquitous in 1950s consumer electronics. The OEM diagram shows a pair of drivers connected in parallel.
O/P "iron" is a BIG deal. It's heavy. So, long distance shipping of otherwise modestly priced items can turn out to be no bargain, at all.
"Local" suppliers frequently wind up costing you less, in toto.The UK trafo winders I'm familiar with are Sowter and Stevens & Billington. Sowter's model U063 has the correct impedances, but its power handling capability is questionable. Brian Sowter is a very helpful fellow. Send him an EMail explaining your situation. Something similar to the U063 on the next larger size laminations is a distinct possibility. While you are at it, consider ultralinear taps.
I'd like to think that the circuitry under discussion is highly competent. So, investing in decent O/P trafos will pay off.
BTW, that 1.5 Ohm impedance you came up with makes sense. Paper cone speaker drivers with a 3.2 Ohm impedance were ubiquitous in 1950s consumer electronics. The OEM diagram shows a pair of drivers connected in parallel.
Thanks Eli, I will look into these brands, unfortunately first impressions are that the Sowter transformers are a lot more expensive than I was hoping, even the 4W ones are over £90 each!
Hammond and Edcor OTs seem more in my price range, but I seem very restricted in my choice of OTs with an input impedance of 15-16kOhm. Can lower impedance OTs be used?
Hammond and Edcor OTs seem more in my price range, but I seem very restricted in my choice of OTs with an input impedance of 15-16kOhm. Can lower impedance OTs be used?
NFB is a tool, not a magic wand. 
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