The DC filaments are 22V @ 300 mA. 4-12V tubes in series-parallel, or 4-6V tubes in series. Since the 12aX7 can run either way, 2 6922s and two 12AX7 could be run in series too.
As for high line voltage, a pair of Keystone inrush limiters on the primary will drop a few volts. The run HOT, keep 'em away from other parts and wiring.
As for high line voltage, a pair of Keystone inrush limiters on the primary will drop a few volts. The run HOT, keep 'em away from other parts and wiring.
All,
Tom B. is correct. The DC heater supply is "25" V./300 mA. My focus was on 2X series pairs in parallel and that's too narrow.
OK, 4X 6.3 V./300 mA. bottles in series work. A 12AX7 and a 6922 for the phono section combined with the 2 splitter/driver tubes looks good. FWIW, I'd still prefer that a UCC88 be combined with a 7058, as both twin triodes have internal shields that could aid the channel separation cause. Both tubes are in both channels.
The ECC808/6KX8 is interesting, but the TFK data sheet shows a 340 mA. heater draw.
Tom B. is correct. The DC heater supply is "25" V./300 mA. My focus was on 2X series pairs in parallel and that's too narrow.
OK, 4X 6.3 V./300 mA. bottles in series work. A 12AX7 and a 6922 for the phono section combined with the 2 splitter/driver tubes looks good. FWIW, I'd still prefer that a UCC88 be combined with a 7058, as both twin triodes have internal shields that could aid the channel separation cause. Both tubes are in both channels.
The ECC808/6KX8 is interesting, but the TFK data sheet shows a 340 mA. heater draw.
Hello Tom,
You are correct sir..thanx for pointing that out . I am still chewing on the reg'ed 6v line possibility though..but you've pointed out a fallback position for me if it becomes too impractical to implement the seperate 6v route.
My thinking is this..
1) If I can re-alocate the bulb current to run BOTH tubes of the phono section, I'll be able to use the 4 dc "slots" in the original design for the remaining line tubes, eliminating all line level ac filaments.
2) Potentially more importantly, I understand (i think) that elevating the filament helps reduce noise (helps prevent the heater from acting like a cathode..right ?). This is most critical in the phono stage. Not only can I not do this with the stock supply,
the reverse is true..the first tube in the string is -22v from ground, and the second is -12V.
Eli,
Thanx for continuing to bat this around with me.
Got it.
(6.3*1.414)*109%(increase from 110v nominal)=9.7 9.7-1(schotky Vd)=8.7 8.7-6.3=2.4 max reg. dropout.
This is a slim margin to be sure, but I may be able to find a reg. that will work....any suggestions.
If I can get the reg'ed 6v worked out, I'm thinking I can run a second bridge off the bias supply to give me a +24v filament supply bias.
As for the current, the dial lamps I've ran into are 3W, on a 6v supply this works out to around 500mA each, times three that gives me 1.5A..with your recommended safety margin, I can run 2 300mA tubes.
What have i missed...will this work ?
Thanx,
Casey
You are correct sir..thanx for pointing that out
. I am still chewing on the reg'ed 6v line possibility though..but you've pointed out a fallback position for me if it becomes too impractical to implement the seperate 6v route.My thinking is this..
1) If I can re-alocate the bulb current to run BOTH tubes of the phono section, I'll be able to use the 4 dc "slots" in the original design for the remaining line tubes, eliminating all line level ac filaments.
2) Potentially more importantly, I understand (i think) that elevating the filament helps reduce noise (helps prevent the heater from acting like a cathode..right ?). This is most critical in the phono stage. Not only can I not do this with the stock supply,
the reverse is true..the first tube in the string is -22v from ground, and the second is -12V.
Eli,
Thanx for continuing to bat this around with me.
Got it.
(6.3*1.414)*109%(increase from 110v nominal)=9.7 9.7-1(schotky Vd)=8.7 8.7-6.3=2.4 max reg. dropout.
This is a slim margin to be sure, but I may be able to find a reg. that will work....any suggestions.
If I can get the reg'ed 6v worked out, I'm thinking I can run a second bridge off the bias supply to give me a +24v filament supply bias.
As for the current, the dial lamps I've ran into are 3W, on a 6v supply this works out to around 500mA each, times three that gives me 1.5A..with your recommended safety margin, I can run 2 300mA tubes.
What have i missed...will this work ?
Thanx,
Casey
No sir. 6.3 * 1.09 = 6.87 6.87 - 1 = 5.87 5.87 * root_2 = 8.3 You are OK with 6 VDC on the heaters. That leaves a THIN margin of 2.3 V. With a low dropout regulator you could be OK.
There is (IMO) a better way to skin this cat. Use 2X Schottky diodes as a "full wave" doubler. Use 2X 1000 muF./15 WVDC 'lytics in the doubler stack. Now, a low cost 7812 regulator IC has enough headroom to function well. Wire 2X 6.3 V/300 mA. heaters in series or 2X 12.6 V./150 mA heaters in parallel. Notice that the energy stays the same, but it's distributed differently. BTW, if you use dropping resistors to control the AC heater voltage, connect the doubler directly to the filament winding. Extra headroom for the regulator IC CAN'T hurt.
I still prefer a 7058/UCC88 combo for the phono stage (internal shields), but by using LEDs for illumination, you do get to put DC on 2 more heaters.
Oopsies...seems I did things in the wrong order...thanx for the correction
I like it..thanx again. On the filament V drop resistors for the AC bottles, do I want to put a resistor on each leg to keep it balanced, or does it matter ?
It's a question of sourcing for me , the market is flush with ECC83/ECC88 variants. Its bad enough with only 2 sources of 7591's. I'll check out the 7058/UCC88 source options though, the internal shields are appealing.
Onto the line/tone stage. (I'll be re-posting some of the schematic from the Sams..much clearer)...
...Do I really need 2 stages (I/P O/P) to drive the the Baxendahl ? One of the appealing features of Thorsten's phono stage is the fact it can drive a low load (10k according to Thorsten...you out there Kuei ? I'd love to get your take on this), this would allow me to shunt input to the tone circuit with a suitably low (47k?) resistor, giving the phono stage a stable load. That seems to me would eliminate the need for the first stage, I assume the second stage is to offset insertion loss (I'm guessing here). If this is the case, it would free up a triode section. I would then be able to run two 12ax7 sections in parallel for the 1st stage of the phono, dividing my noise by 2 in this critical stage( it would however, put the second phono stage back on the negatively biased filament supply..oh well, this is a world of compomises, isn't it). The second stage would only be in line when the tone is switched in, switched out, the amp is following the rule of 2's. Two phono stages, and 2 PA stages (one Schmidt splitter/VA stage and the outputs).
This is starting to look good (in my head anyway).
Thoughts?
Casey
Casey,
Except for the heater (12.6 vs. 6.3), the UCC88 and the ECC88 are the same tube. Both have an internal shield connected to pin 9. Some scrounging will be required to come up with UCC88s.
The 7058 is a different story. GE built 7058s are under $10 from AES. RCA built tubes will cost more.
RES lists a price of $46 for a 6KX8. That will be either a TFK or a Mullard. Getting 1 of those into the phono section is worth a struggle. Perhaps a LT brand low dropout regulator IC would be OK with the limited headroom available. Check out the data sheets. You'd need 640 mA. at 6 V. for a 6KX8/6922 combo in the phono stage.
Parallel 'X7 sections forces a recalculation of the RIAA network. NYET!
DC on the phono section heaters is plenty quiet. IMO, biasing off B+ is not necessary. Also, you have less ground watching to do.
Except for the heater (12.6 vs. 6.3), the UCC88 and the ECC88 are the same tube. Both have an internal shield connected to pin 9. Some scrounging will be required to come up with UCC88s.
The 7058 is a different story. GE built 7058s are under $10 from AES. RCA built tubes will cost more.
RES lists a price of $46 for a 6KX8. That will be either a TFK or a Mullard. Getting 1 of those into the phono section is worth a struggle. Perhaps a LT brand low dropout regulator IC would be OK with the limited headroom available. Check out the data sheets. You'd need 640 mA. at 6 V. for a 6KX8/6922 combo in the phono stage.
Parallel 'X7 sections forces a recalculation of the RIAA network. NYET!
DC on the phono section heaters is plenty quiet. IMO, biasing off B+ is not necessary. Also, you have less ground watching to do.
Casey,
The triodes in the 6KX8 are electrically equivalent to those in the 12AX7. However the 6KX8 is "6" V. only and it doesn't pin out the same as the 'X7. The key feature of the 6KX8 is its internal shield. 6KX8/ECC808 Data Sheet
The triodes in the 6KX8 are electrically equivalent to those in the 12AX7. However the 6KX8 is "6" V. only and it doesn't pin out the same as the 'X7. The key feature of the 6KX8 is its internal shield. 6KX8/ECC808 Data Sheet
rcavictim said:
I really need to get some reading glasses that work. When I glanced at the posted schematic I saw a bridge rectifier followed by a CLC filter and assumed that was the B+ supply. Turned out I was seeing the DC filament supply. Hence my above post. Now I have seen my error and can now accuse Fisher also of using a stupid voltage doubler like others did here, of which Sherwood came first to mind.
When does one normally use a doubler? How about either not willing to spend coin on a real power transformer, or maybe no transformer at all (ultra tight wad married to Miss. Series String Filaments.) or has a serious hate relationship with electrolytic caps and all he really wanted in real life was job as an executioner.
rcavictim,
Dude..your'e harshing my high man
First EH tubes are dry cat scat turning white in the lawn, and now ALL amps with doublers are nothing more than consumer tripe, not worthy of consideration...sheesh.
Hey, I would prefer a proper secondary driving a full bridge as well, but its not in the offering. I am looking into ways to improve what I have, a full-wave doubler perhaps ?
Casey
Dude..your'e harshing my high man
First EH tubes are dry cat scat turning white in the lawn, and now ALL amps with doublers are nothing more than consumer tripe, not worthy of consideration...sheesh.
Hey, I would prefer a proper secondary driving a full bridge as well, but its not in the offering. I am looking into ways to improve what I have, a full-wave doubler perhaps ?
Casey
valveitude said:
Casey,
We need to find the tone control circuit schematic I previously mentioned. IIRC, it uses 'X7s as buffers. Also, you'd never have a problem selling NOS TFK smoothplate ECC83s, as even used smoothplates are marketable.
Think in terms of the number of 9 pin sockets available. Use 12AT7s for the Schmitt splitter/drivers. The 7591 is EASY to drive. Both Fisher and Scott took advantage of that fact. A 5965 would have enough gain, if loop NFB wasn't part of the equation. Good sounding 'T7s are reasonably plentiful. 1950s vintage black plate RCA 6201s are about $25, as are 1950s vintage Brimar black plate 'T7s. There are several Mullard versions that don't cost body parts. Sylvania Gold Brand is NICE too.
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Bob,
There's nothing wrong with a doubler B+ PSU, IF the details are "sweated". The H/K Cit. 2 is an excellent example, as is Poindexter's "Musical Machine". Plenty of capacitance in the doubler stack followed by a low DCR choke and more capacitance get the job done. As always, TANSTAAFL applies.
Hi valveitude,
Work with what you have. My Eico (another one Rob ) has a voltage doubler for the B+. I can't do anything about this. It does sound great, but then I don't pound it either.
Rob, nothing has changed in consumer electronics except that construction is even more marginal now. Can you imagine a surround receiver lasting this long and being worth rebuilding.
Hmmmmm ........ don't answer that. I remember looking at a Sony KSS-150A transport thinking "what garbage!". Today, it would be a high end transport.
How low can you go?
-Chris
Work with what you have. My Eico (another one Rob
) has a voltage doubler for the B+. I can't do anything about this. It does sound great, but then I don't pound it either.Rob, nothing has changed in consumer electronics except that construction is even more marginal now. Can you imagine a surround receiver lasting this long and being worth rebuilding.
Hmmmmm ........ don't answer that. I remember looking at a Sony KSS-150A transport thinking "what garbage!". Today, it would be a high end transport.
How low can you go?-Chris
anatech said:Hi valveitude,
Work with what you have. My Eico (another one Rob
A voltage doubler in your tubed Dynaco?!!!!
Oh man. My HF-20's use a conventonal center tapped winding two diode (tube) rectifier full wave supply. Unfortunately C-R-C (no choke) after that. At least I have room to shoe horn in a choke if I wanted.Hmmm, good point Chris.
Hmmmmm ........ don't answer that. I remember looking at a Sony KSS-150A transport thinking "what garbage!". Today, it would be a high end transport.
-Chris [/B]
valveitude said:rcavictim,
Dude..your'e harshing my high man
First EH tubes are dry cat scat turning white in the lawn, and now ALL amps with doublers are nothing more than consumer tripe, not worthy of consideration...sheesh.
Hey, I would prefer a proper secondary driving a full bridge as well, but its not in the offering. I am looking into ways to improve what I have, a full-wave doubler perhaps ?
Casey
Sorry Casey, That 2nd statement bit about the voltage doubler was in annoyance of me discovering that I'm going blind.
To help you understand my distaste of voltage doublers, I purchased a brand new stereo tube amp from China not long ago with SE 805's that employed an innapropiate voltage doubler in a high power, high voltage circuit and the voltage doubler idea caused the early death of the amp (after just 5 hours use) which included toasting a very expensive to replace 10 winding, interwinding shielded power transformer. This new amp cost me more money than I had ever spent on a store bought stereo system component before in my life. I was not particularly impressed. I frequently spend more on parts alone for my DIY projects though, but they don't blow up. This otherwise impressive amplifier got completely evaluated, remanufactured and now has a big Hammond plate xfmer in it, full wave bridge and proper choke input B+ supply, as well as many other major power supply improvements. Saved by being able to DIY, I ended up with a really, really good amplifier after months of work.
I have heard a 500C and I liked the tone factory stock so I know you have a good tube receiver there. to improve and enjoy!
Eli,
Whats this "WE" stuff . Tell me what you know, and I'll spend some time searching for it.
Ok, help clear this up for me. What is the advantage of using a 12AT7 over a 12AX7 pair ? If the 12AX7 is resonable, I think I would like the extra gain..of course I don't know if the 12AT7 has enough gain to drive the amp to full power with Thorsten's phono pre. Since the Schmidt will be more linear than the OEM circuit, I wont need/want as much feedback for the same THD.
I was a little confused with the above comment..is the 5965 a differnt tube, or is it a 12AT7 variant ?
Thanx again Eli..youve been a BIG help.
Hello Anatech,
Not to worry. My last Amp (many moons ago) was a reworked HK Citation V. I am well aware of what a properly implemented doubler is capable of. The Citation had its short comings but it was THE ballziest tube amp I ever heard in it's power class. I hooked it up to a pair of Apogee Scintillas (200 watt recommended minimum, if I remember correctly) and it played them to a surprising level..it shamed a NAD 2240 (120/w chn SS)power amp with the same load.
Oh.. I DID pound on it..never so much as a blown fuse
Casey
Whats this "WE" stuff
. Tell me what you know, and I'll spend some time searching for it.Ok, help clear this up for me. What is the advantage of using a 12AT7 over a 12AX7 pair ? If the 12AX7 is resonable, I think I would like the extra gain..of course I don't know if the 12AT7 has enough gain to drive the amp to full power with Thorsten's phono pre. Since the Schmidt will be more linear than the OEM circuit, I wont need/want as much feedback for the same THD.
I was a little confused with the above comment..is the 5965 a differnt tube, or is it a 12AT7 variant ?
Thanx again Eli..youve been a BIG help.
Hello Anatech,
Not to worry. My last Amp (many moons ago) was a reworked HK Citation V. I am well aware of what a properly implemented doubler is capable of. The Citation had its short comings but it was THE ballziest tube amp I ever heard in it's power class. I hooked it up to a pair of Apogee Scintillas (200 watt recommended minimum, if I remember correctly) and it played them to a surprising level..it shamed a NAD 2240 (120/w chn SS)power amp with the same load.
Oh.. I DID pound on it..never so much as a blown fuse
Casey
Hi Rob,
You know, I find more design problems in new gear than anything I've pulled apart from the 50's through 60's (tube, not counting TV's).
Sorry to hear about your amp. I'm sure it really got to you!
Casey, just smile and rebuild it. There is time for rebuilding sections later. Get it working first. Then, you can actually see if something is an improvement.
-Chris
You know, I find more design problems in new gear than anything I've pulled apart from the 50's through 60's (tube, not counting TV's).
Sorry to hear about your amp. I'm sure it really got to you!
Casey, just smile and rebuild it. There is time for rebuilding sections later. Get it working first. Then, you can actually see if something is an improvement.
-Chris
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