A quick note about the cathode by-pass capacitor in this amp.....
Pertaining ONLY to Push-Pull Class A circuit as such....
In "theory" you would not need a bypass cap in an ideal situation..since there would be no AC signal to bypass at the cathode with perfect Class A signal balance at this node...
In reality, you need this bypass since you do not have perfect AC balance... Not to be confused with DC balance.... You can have perfect DC balance, while still having imbalance in the AC, thus the gm not perfectly balanced in large signal operation....while the valves may "appear" gm matched in a tester which onlt test small signal operation....
A bloody big bypass cap enures a better phase margin in the low frequency response, despite the more than adequate time constant way below 20Hz...
Just me 2 cents
Chris
Pertaining ONLY to Push-Pull Class A circuit as such....
In "theory" you would not need a bypass cap in an ideal situation..since there would be no AC signal to bypass at the cathode with perfect Class A signal balance at this node...
In reality, you need this bypass since you do not have perfect AC balance... Not to be confused with DC balance.... You can have perfect DC balance, while still having imbalance in the AC, thus the gm not perfectly balanced in large signal operation....while the valves may "appear" gm matched in a tester which onlt test small signal operation....
A bloody big bypass cap enures a better phase margin in the low frequency response, despite the more than adequate time constant way below 20Hz...
Just me 2 cents
Chris
I'm a bit suprised to hear about heat problems on Quad 2, as similar amp designs were used on the Bell & Howell projectors around in the late 50'S. These were basically a "sardine tin" version of the existing amp using pair KT66's crammed into a restricted guts of a B&H cine projector. A point to bear in mind that ambient temp room levels have increased by perhaps 10°C since the birth of Quad 2, so suspect overheating comments don't come as a suprise.
Evoking old memories....Has anyone got such a B&H "priceless reel gem"...........?
richj
Evoking old memories....Has anyone got such a B&H "priceless reel gem"...........?
richj
It can be hard to find reliable data about the distance between 2 valves because the lack of good data sheet in many cases.
This data is not in the data sheet I have in my possession for the KT66. But I have some other. In the EL34 Mullard data sheet, I can read "As a rough guide, the distance between two EL34's could well be at least 40 mm, and the distance from the cabinet and other componants at least 30 mm"
It is not the case with the QuadII, the two KT66 are very close to each other and very close the other componants.
It is another heat cause in the QuadII, the main transformer. If the output transformer have a very good quality, the main transformer is of a very cheap model. It warm a lot, and with time and the heat, the isolation, some kind of black-brown thing, go out the transformer and it still work, but heat more.
I have a quad stereo with 2 of those amps. One of the amp was talking more as 200 watts on the main witout the tubes on it. I mean just to power on the main transformer. You can figure out the heat.
I have buy 2 transformers on a local manufacturer, one for each of those amp, they are not on the amps but down at the floor, and even the other amp is becoming not so warm as before. I can even say at the 2 amps are a lot cooler as before.
In the original users manual of those amps, Quad was writing at after a few hours (3 or 4, I don't remember exaclty, it is in a box...) of operation, you must shut down them for minimum the same amount of time. They wrote this because they was knowing at this amp is heating too much.
It was a relatively very cheap amp at this time. To archive a so low cost, they have spare money, on the main transformer, on the size, on the caps. The caps have a good quality, but they have very low values. You can archive a sound in the bass with a much better dynamic just by replacing all the cathode and bypass caps with higher value.
This data is not in the data sheet I have in my possession for the KT66. But I have some other. In the EL34 Mullard data sheet, I can read "As a rough guide, the distance between two EL34's could well be at least 40 mm, and the distance from the cabinet and other componants at least 30 mm"
It is not the case with the QuadII, the two KT66 are very close to each other and very close the other componants.
It is another heat cause in the QuadII, the main transformer. If the output transformer have a very good quality, the main transformer is of a very cheap model. It warm a lot, and with time and the heat, the isolation, some kind of black-brown thing, go out the transformer and it still work, but heat more.
I have a quad stereo with 2 of those amps. One of the amp was talking more as 200 watts on the main witout the tubes on it. I mean just to power on the main transformer. You can figure out the heat.
I have buy 2 transformers on a local manufacturer, one for each of those amp, they are not on the amps but down at the floor, and even the other amp is becoming not so warm as before. I can even say at the 2 amps are a lot cooler as before.
In the original users manual of those amps, Quad was writing at after a few hours (3 or 4, I don't remember exaclty, it is in a box...) of operation, you must shut down them for minimum the same amount of time. They wrote this because they was knowing at this amp is heating too much.
It was a relatively very cheap amp at this time. To archive a so low cost, they have spare money, on the main transformer, on the size, on the caps. The caps have a good quality, but they have very low values. You can archive a sound in the bass with a much better dynamic just by replacing all the cathode and bypass caps with higher value.
I agree with Frank.....this isn't a "British design" in having to shut an amp down because it's overheating. I recon Dominic has been given a clone Quad 2 .....not the original British masterpiece.
If he is short of details about KT tube orientation and spacing then it's in GEC app rep No3 printed 1976. I cannot paste this as I don't have facilities.
richj
If he is short of details about KT tube orientation and spacing then it's in GEC app rep No3 printed 1976. I cannot paste this as I don't have facilities.
richj
fdegrove said:Hi,
That's the first time I hear about that.
Cheers,
You are right, I have searched the manual in my boxes and they doesn't write was I was telling. I was confounding it with something else.
But it remain at my Quad, even if it is an original english made one, is heating much less now as before. I have a friend that have the same, but in original state, and it become much warmer as mine after a few hours.
Thank you Richj for the information, I have found the data sheet for the GEC KT66, and they wrote at "a pair of valves working at maximum ratings should be mounted at no less than 9cm (3.5 in.) between centers."
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Valve-Data/Valve-data.html
Interesting post here.
I think at least part of the heat problem lies with the proximity of the KT66s and GZ32 to the power transformer and choke. Mounting the KT66 cathode resistor right under the choke (and close to the byupass electrolytic!) did not help either and the chokes usually oozed out first. I noticed that with certain Russian KT66s in the Coke-shaped envelope, the glass thus quite closer to the components, it got quite hotter than with smaller 6L6s. (OK, the heater current was less but only by 5W, but it is not a lot on a 90W transformer.) I also replaced the GZ32 (Coke-bottle shape) with the physically smaller GZ34 and in that case the choke was cooler and power transformer was still cooler.
I have 4 Quads with me (refurbished for a customer) and will test in the near future without the valves, with dummy loads. I have replaced one power transformer with a low-loss C-core one; comparison will be interesting.
At a stage in my career we decided that potting transformers in an enclosure made them considerably hotter measured on the windings (increase in copper resistance was measured), compared to laquer impregnating them and then mounting air-surrounded in the same enclosure.
While posting, may I ask if anyone has experience of the 2 new Quad editions? The "Classic" looks exactly the same (also inside) as the original, while component placement in the 40W version is quite different. The latter was apparently "redesigned" (meaning what?) by Andy Grove (any relation, Frank?) but as far as I could see the circuit was the same - some necessary adjustment in resistor values. When asking the manufacturers in the UK about this I got vague non-technical replies and was promised a cuircuit diagram which never arrived.
One matter I did find curious was the replacement of the EF86s with 6SG7s, which were already declared obsolete (by RCA) in the 60s! Admittedly the Quad 40 tubes were military spec. but still.........
Perhaps someone would care to comment.
Regards.
I think at least part of the heat problem lies with the proximity of the KT66s and GZ32 to the power transformer and choke. Mounting the KT66 cathode resistor right under the choke (and close to the byupass electrolytic!) did not help either and the chokes usually oozed out first. I noticed that with certain Russian KT66s in the Coke-shaped envelope, the glass thus quite closer to the components, it got quite hotter than with smaller 6L6s. (OK, the heater current was less but only by 5W, but it is not a lot on a 90W transformer.) I also replaced the GZ32 (Coke-bottle shape) with the physically smaller GZ34 and in that case the choke was cooler and power transformer was still cooler.
I have 4 Quads with me (refurbished for a customer) and will test in the near future without the valves, with dummy loads. I have replaced one power transformer with a low-loss C-core one; comparison will be interesting.
At a stage in my career we decided that potting transformers in an enclosure made them considerably hotter measured on the windings (increase in copper resistance was measured), compared to laquer impregnating them and then mounting air-surrounded in the same enclosure.
While posting, may I ask if anyone has experience of the 2 new Quad editions? The "Classic" looks exactly the same (also inside) as the original, while component placement in the 40W version is quite different. The latter was apparently "redesigned" (meaning what?) by Andy Grove (any relation, Frank?) but as far as I could see the circuit was the same - some necessary adjustment in resistor values. When asking the manufacturers in the UK about this I got vague non-technical replies and was promised a cuircuit diagram which never arrived.
One matter I did find curious was the replacement of the EF86s with 6SG7s, which were already declared obsolete (by RCA) in the 60s! Admittedly the Quad 40 tubes were military spec. but still.........
Perhaps someone would care to comment.
Regards.
Hi,
Twenty years after the decision was made?
In those days there were still plenty of ex-USSR EF86s around and Svetlana did a couple of re-runs after that as well.
Either way, I recall I was a bit puzzled by that decision back than.
The main problem with some of the old QIIs is the fact they were using potted trannies and those had some weird tar based compound that tended to leak out when things got too hot.
My advice is, if you don'y have that problem leave them alone and put them in the open, not in a sparsely vented cabinet and you'll be fine.
Modding them is fun but not really worth it unlesss you're really enamoured with the old things perhaps.
They were nice with the old Q ESLs though.
Other than a couple of pints more than 20 years back I don't thinks so.
My last name is preceded by De so it may look similar but etymologically it isn't the same.
No expereince with the new valved Quads I'm afraid.
Cheers,
Twenty years after the decision was made?
In those days there were still plenty of ex-USSR EF86s around and Svetlana did a couple of re-runs after that as well.
Either way, I recall I was a bit puzzled by that decision back than.
The main problem with some of the old QIIs is the fact they were using potted trannies and those had some weird tar based compound that tended to leak out when things got too hot.
My advice is, if you don'y have that problem leave them alone and put them in the open, not in a sparsely vented cabinet and you'll be fine.
Modding them is fun but not really worth it unlesss you're really enamoured with the old things perhaps.
They were nice with the old Q ESLs though.
Other than a couple of pints more than 20 years back I don't thinks so.
My last name is preceded by De so it may look similar but etymologically it isn't the same.
No expereince with the new valved Quads I'm afraid.
Cheers,
Of course, Frank! Sorry!
I have just wiped my spectacles so that I can see names more clearly.
EC8010,
Sadly, no. As far as I could trace with valve sockets and most of the wiring and components visible the circuit was the same as before. I unfortunately did not write down the changed values; I believe the pentode anode loads were 220K, but do not regard that as fact. The bias resistors were also different, but the feedback ratio appeared the same. The h.t. I think is 410V.
I was entertained by the local agents at the time (about 18 months ago) because I wrote a technical analysis of the amplifier in a local hi-fi magazine. The promised circuit diagram is still owed me, so perhaps I could twist an arm there. I did quite a write-up, so they should still feel friendly! Let us see.
The Quad 40s are quite expensive by our levels and I do not believe will be popular here for that reason.
I have just wiped my spectacles so that I can see names more clearly.
EC8010,
Sadly, no. As far as I could trace with valve sockets and most of the wiring and components visible the circuit was the same as before. I unfortunately did not write down the changed values; I believe the pentode anode loads were 220K, but do not regard that as fact. The bias resistors were also different, but the feedback ratio appeared the same. The h.t. I think is 410V.
I was entertained by the local agents at the time (about 18 months ago) because I wrote a technical analysis of the amplifier in a local hi-fi magazine. The promised circuit diagram is still owed me, so perhaps I could twist an arm there. I did quite a write-up, so they should still feel friendly! Let us see.
The Quad 40s are quite expensive by our levels and I do not believe will be popular here for that reason.
fdegrove said:Hi,
The main problem with some of the old QIIs is the fact they were using potted trannies and those had some weird tar based compound that tended to leak out when things got too hot.
My advice is, if you don'y have that problem leave them alone and put them in the open, not in a sparsely vented cabinet and you'll be fine.
Cheers,
Hi,
My 2 Quads II are such oldies. What I did was to left the original trannies on the amps, so their still have the original look, remove the main plug and install a cable going thru the cabinet at the place of the main plug to the trannies. The main switch on the quad22 have no effect anymore and I have installed another switch on the front of the "rack" where I have the whole stereo.
I changed the GZ32s against GZ34s and the 180 ohms cathode resistors against 240 ohms, that in order to accomodate the higher +B. I changed C5 25uF against 1000uF and 470nF in parallel, and C2+C3 was changed from 100nF to 470nF. The result is at the sound is as good as before, but I get around 25 watts output and a much cleaner sound in the bass. I write around because I have not mesured the distortion so I am not sure at my mesurment correspond to the original mesurment.
Ciao
The main issue is at you are using high quality caps. As electrolytics are no so good with high frequency, if you increase it, you have to put a lower cap in parallel.
The problem here is if you want the best possible sound with the QII, you must have an amplifier that will have enough bandwitch without feedback to reproduce the whole frequency range, and still will not oscillate when the feedback is applied.
Feedback is good to linearize the gain, but not to fix the bandwitch.
I have post a file with the basic equations for the basic vacuum circuits.
http://www.diyaudio.com/forums/showthread.php?s=&postid=841168#post841168
http://z26.zupload.com/download.php?file=getfile&filepath=4614
I believe at it come from the Aiken website, but it is down for now.
It is the same problem with C2 +C3. Here you must use high quality caps too. It is special caps for audio tubes, they are black and very expensive. Otherwise, caps for horizontal deflection of TV sets are very good too, like MKT or MKP.
I have done the same modifications in the preamp too. I have changed C3 +C4 against 470nF and the cathode caps C5 +C6 against higer values. But I don't remember exactly the value of the cathode caps because I am living now in a temporary house, and big parts of my stuffs are in boxes in the basement. A women problem...
I don't remember either what I have done with C10 + C11 and C13 + C14.
A good thing to do is to add two 470nF caps in serie with the volume potentiometer RV1A and RV1B, if it is not allready implemented. It will do at, if a tuner or CD, is leaking some DC current, you will not get it to the grid of the first triode.
But you must don't change anything else in the preamp, especialy not the caps in the filter. The caps C16 + C17 are part of the filter, so you don't have to change it.
The problem here is if you want the best possible sound with the QII, you must have an amplifier that will have enough bandwitch without feedback to reproduce the whole frequency range, and still will not oscillate when the feedback is applied.
Feedback is good to linearize the gain, but not to fix the bandwitch.
I have post a file with the basic equations for the basic vacuum circuits.
http://www.diyaudio.com/forums/showthread.php?s=&postid=841168#post841168
http://z26.zupload.com/download.php?file=getfile&filepath=4614
I believe at it come from the Aiken website, but it is down for now.
It is the same problem with C2 +C3. Here you must use high quality caps too. It is special caps for audio tubes, they are black and very expensive. Otherwise, caps for horizontal deflection of TV sets are very good too, like MKT or MKP.
I have done the same modifications in the preamp too. I have changed C3 +C4 against 470nF and the cathode caps C5 +C6 against higer values. But I don't remember exactly the value of the cathode caps because I am living now in a temporary house, and big parts of my stuffs are in boxes in the basement. A women problem...
I don't remember either what I have done with C10 + C11 and C13 + C14.
A good thing to do is to add two 470nF caps in serie with the volume potentiometer RV1A and RV1B, if it is not allready implemented. It will do at, if a tuner or CD, is leaking some DC current, you will not get it to the grid of the first triode.
But you must don't change anything else in the preamp, especialy not the caps in the filter. The caps C16 + C17 are part of the filter, so you don't have to change it.
EC8010,
Will sure do when I have it. But since this is a thoroughly international site, I am wondering whether some of our British friends cannot obtain this easier.
Some further remarks:
Regarding the cathode bypass, a few 1000uF would not do harm. Just keep in mind that large electrolytics tend to be less than ideal. It is better to group a number of 470s together, if this does not begin to look grotesque, plus a 100nF or such.
Increasing the bias resistor to 240 ohm might let you end in slightly class AB - not saying that is necessarily an evil, but the OPT was designed for class A, plus it is rather a 15W type. Some 3rd harmonic distortion could occur at 23W at lowest notes.
The time constant for C2.R7/C3.R9 is already only 2,3 Hz; why does one want to still decrease that by increasing C2/C3? The screen time constant (as a result of gain from G2s) is somewhat higher, but still below 8 Hz. And I am not sure why exotic capacitors are necessary - polyesters are as good as one will need. (I had a limited budget all my life!)
Regarding the pre-amp (Quad22) my experience has been rather different. Again the V1/V2 time constants are below 10 Hz. But here I made a greater change. To me practice dictated that all those many contacts on switch and plug-ins can be a source of trouble and sometimes was. We no longer require all the many equalising possibilities - there is only RIAA, and most MM cartridges wil output 2,5mV or so. And I just could not get the required response out of this circuit, with all respect to Dr Walker (with whom I had corresponded and has great respect for). I therefore replaced the EF86 with an ECC83 equalising circuit with a nett gain of about 30, also allowing me to slash response below 23 Hz at a handy 18 dB/octave. The need for some form of rumble filter was apparently felt even by Quad, by tayloring C13 and C16 in later models (the latter as D_F mentioned).
I am the first to concur that that might decrease the "vintage value", but the decision to me (and my clients) was whether one wanted it for that or a proper response. And.....
There is a further serious problem at which I hinted before, and that is that in many models the capacitors C18 - C25 inclusive drifted by some 200 - 300% (no typo). (That is mainly why Quad 22s have been reported to sound crappy in recent times.) In my cases I had to replace all, also changing C18 - C21 to 4.7nF and C22 - C25 to 3.3nF to get close to the published graphs. I have had experience of 8 Quad 22s, and others have also reported this flaw. It might be though that some may have stayed "in tune", depending on date of manufacture.
Apology for the lengthy post; this will hopefully be of value to some. I will appreciate experiences by other honourable members.
Will sure do when I have it. But since this is a thoroughly international site, I am wondering whether some of our British friends cannot obtain this easier.
Some further remarks:
Regarding the cathode bypass, a few 1000uF would not do harm. Just keep in mind that large electrolytics tend to be less than ideal. It is better to group a number of 470s together, if this does not begin to look grotesque, plus a 100nF or such.
Increasing the bias resistor to 240 ohm might let you end in slightly class AB - not saying that is necessarily an evil, but the OPT was designed for class A, plus it is rather a 15W type. Some 3rd harmonic distortion could occur at 23W at lowest notes.
The time constant for C2.R7/C3.R9 is already only 2,3 Hz; why does one want to still decrease that by increasing C2/C3? The screen time constant (as a result of gain from G2s) is somewhat higher, but still below 8 Hz. And I am not sure why exotic capacitors are necessary - polyesters are as good as one will need. (I had a limited budget all my life!)
Regarding the pre-amp (Quad22) my experience has been rather different. Again the V1/V2 time constants are below 10 Hz. But here I made a greater change. To me practice dictated that all those many contacts on switch and plug-ins can be a source of trouble and sometimes was. We no longer require all the many equalising possibilities - there is only RIAA, and most MM cartridges wil output 2,5mV or so. And I just could not get the required response out of this circuit, with all respect to Dr Walker (with whom I had corresponded and has great respect for). I therefore replaced the EF86 with an ECC83 equalising circuit with a nett gain of about 30, also allowing me to slash response below 23 Hz at a handy 18 dB/octave. The need for some form of rumble filter was apparently felt even by Quad, by tayloring C13 and C16 in later models (the latter as D_F mentioned).
I am the first to concur that that might decrease the "vintage value", but the decision to me (and my clients) was whether one wanted it for that or a proper response. And.....
There is a further serious problem at which I hinted before, and that is that in many models the capacitors C18 - C25 inclusive drifted by some 200 - 300% (no typo). (That is mainly why Quad 22s have been reported to sound crappy in recent times.) In my cases I had to replace all, also changing C18 - C21 to 4.7nF and C22 - C25 to 3.3nF to get close to the published graphs. I have had experience of 8 Quad 22s, and others have also reported this flaw. It might be though that some may have stayed "in tune", depending on date of manufacture.
Apology for the lengthy post; this will hopefully be of value to some. I will appreciate experiences by other honourable members.
David, Caio and All
A few quick points form my limited experience of QUAD II
- take them or leave them -
GZ34 in place of GZ32 can give about 15-20V extra HT with corresponding increase in output power and output valve dissipation. DO NOT use solid state replacement rectifier without series resistors to keep the HT to about 340V.
If your mains voltage is high as it is here in the UK (and parts Europe >240V a.c.) increasing R12 is a good option to reduce the output valve dissipation, but increasing C5 much above 47uF has little effect and lower values can be non electrolytic. The heater voltages will still be too high, which reduces valve life, but the gm will be higher which…..?
R12/C5 original time constant is about 40Hz, which is suitable for the output transformer in open loop. Good results (audible) can be obtained when not using 22 control if R1 is lowered and a capacitor placed between input and R1, A typical change - R1=100kR and Cin 75nF (polystyrene of course!) gives a bass roll off at about 20Hz which decreases the TID due to crazy low frequencies on modern source material. Try it - you can always put it back if you sell the amps.
If using a 22 and the on/off switch S1 has failed you can wire S4 – STEREO button as an on/off switch, this way you can leave the volume control set and switch the amp using the STEREO/MON buttons, other tips and schematics etc. on my web site….
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/QUAD/QUAD-Mods-and-info.html
Regards to All
Keith
A few quick points form my limited experience of QUAD II
- take them or leave them -
GZ34 in place of GZ32 can give about 15-20V extra HT with corresponding increase in output power and output valve dissipation. DO NOT use solid state replacement rectifier without series resistors to keep the HT to about 340V.
If your mains voltage is high as it is here in the UK (and parts Europe >240V a.c.) increasing R12 is a good option to reduce the output valve dissipation, but increasing C5 much above 47uF has little effect and lower values can be non electrolytic. The heater voltages will still be too high, which reduces valve life, but the gm will be higher which…..?
R12/C5 original time constant is about 40Hz, which is suitable for the output transformer in open loop. Good results (audible) can be obtained when not using 22 control if R1 is lowered and a capacitor placed between input and R1, A typical change - R1=100kR and Cin 75nF (polystyrene of course!) gives a bass roll off at about 20Hz which decreases the TID due to crazy low frequencies on modern source material. Try it - you can always put it back if you sell the amps.
If using a 22 and the on/off switch S1 has failed you can wire S4 – STEREO button as an on/off switch, this way you can leave the volume control set and switch the amp using the STEREO/MON buttons, other tips and schematics etc. on my web site….
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/QUAD/QUAD-Mods-and-info.html
Regards to All
Keith
Keith,
Let me first congratulate you on a Quad site very neatly and concisely composed (in contrast to some of the stuff found on the web). Our experiences were similar.
I have indicated on another Quad thread (we seem to have had 2 for a time) that I found a 1,5nF phase compensation capacitor across R11 to improve square wave rise response somewhat - that for an 8 ohm output impedance connection, which is the requirement of the day. (This had to do a.o. with the absence of g1-common capacitance of about 20pF when replacing C2-C3, as you have mentioned.) By the way, I see you did not include R11 in your upgraded board photo. You might also consider advising that R12 be mounted away from directly under the choke and so close to C5.
The reason for using a high value cap over R12 is to provide for some approach to fixed bias under music conditions, which will yield a somewhat higher clean peak output for short bursts of music. This is really advantageous in class AB operation, but even in class A, with beam tubes current drawn at full output can be up to 20% higher than quiescent, raising the bias and slightly limiting said output. The effect will not be audible under normal conditions and is marginal, but for the cost of a capacitor(s)...... I have also known folks to replace R12 with suitable zener diodes for real fixed bias, but have never done that myself. This rather requires the output tubes to be matched.
You do not mention the defects I encountered in the pre-amp, with C18-25 having drifted out of all bounds (see my previous post). What was your experience regarding this?
Regards.
Let me first congratulate you on a Quad site very neatly and concisely composed (in contrast to some of the stuff found on the web). Our experiences were similar.
I have indicated on another Quad thread (we seem to have had 2 for a time) that I found a 1,5nF phase compensation capacitor across R11 to improve square wave rise response somewhat - that for an 8 ohm output impedance connection, which is the requirement of the day. (This had to do a.o. with the absence of g1-common capacitance of about 20pF when replacing C2-C3, as you have mentioned.) By the way, I see you did not include R11 in your upgraded board photo. You might also consider advising that R12 be mounted away from directly under the choke and so close to C5.
The reason for using a high value cap over R12 is to provide for some approach to fixed bias under music conditions, which will yield a somewhat higher clean peak output for short bursts of music. This is really advantageous in class AB operation, but even in class A, with beam tubes current drawn at full output can be up to 20% higher than quiescent, raising the bias and slightly limiting said output. The effect will not be audible under normal conditions and is marginal, but for the cost of a capacitor(s)...... I have also known folks to replace R12 with suitable zener diodes for real fixed bias, but have never done that myself. This rather requires the output tubes to be matched.
You do not mention the defects I encountered in the pre-amp, with C18-25 having drifted out of all bounds (see my previous post). What was your experience regarding this?
Regards.
CORRECTION!
In my previous posts I referred to capacitors in the pre-amp that totally drifted from the assigned values. On close scrutiny, these should be C18 - C23 inclusive and C28 and C29 - NOT also C24 and C25 as indicated. My A-4 copy of the diagram is not all that distinct. The values of 3,3nF that I used was for C22, C23, C28 and C29. I did not find problems with C24-25 of 220pF.
Sorry!
In my previous posts I referred to capacitors in the pre-amp that totally drifted from the assigned values. On close scrutiny, these should be C18 - C23 inclusive and C28 and C29 - NOT also C24 and C25 as indicated. My A-4 copy of the diagram is not all that distinct. The values of 3,3nF that I used was for C22, C23, C28 and C29. I did not find problems with C24-25 of 220pF.
Sorry!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.