New Project: 'Junk Parts' Amp build - Voltage / Bias question

Voltage drop at rectifier depends on the current. I'm going from memory. It might be a bit less

My experience is the DC on the B+ doesnt change much if first cap is like 22 uF or larger. Depends on current though.
 
By the way my moto amp uses a 3V rectifier -- a 3DG4, and its voltage drop is right around 15V at the current demanded from my amp, but I believe the 5AR4 drops more than the 3DG4 under similar conditions.
 
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I really don't like tapping the output transformers after a resistor. The more impedance you add to the supply, the more things sag and that does nothing good for power output or distortion. OPT comes straight off the first cap, anything else can go where it needs to. If I have a choice about it, I try to not have the screens after a really big resistor either. Maybe tap that after the choke.
 
FC, in looking at that schematic again I now see what you're referring to with the 12AX and 12AU. Honestly, I missed that originally.....I thought that might be some vestige of the 'pre-amp' section and that all that was necessary was the 12AX portion. I'd mention here that I've planned the amp with only 2 driver tubes.....so I'm going to need a driver section that consists of just that. Be it a 12AX, AU, AT, a 'dis-similar twin-triode like the 7247, or a triode / pentode like the 7199, 6GH8, or even a 6AN8. I guess I'll cross that bridge when I get to it. For now, I just want to get the output tubes lit and 'powered' such that it's drawing current. Once that's done I'll worry about what driver design to incorporate. Given this 'glitch' with the need for the AU (or BH7 as you recommend).... I'll probably go back to either the Rowe driver or something else. The Fisher schematic I have uses one 12AX7 per side...... but I don't recall the model. I've seen what appeared to be this same amp with both 12AX7 and 7247s......so, I guess there are some 'Fisher' deviations that one does need to be careful about.

I just noticed that the Channel Master also uses a 12AU7 in front of a pair of 12AX7s. I think a lot has to do with the strength of your source signal. If its a CD or similar 2v source you might just be able to eliminate that first tube. Lots of amps just use two 12AX7s. I'm listening to a Fisher SA-16 using iTunes and it seems to work fine with that setup. The Maggies and others do fine too.
 
Y'all, et al. C'm on....
Something on your mind, 6?

I really don't like tapping the output transformers after a resistor.
I'd be okay with it as long as the resistor is small. Even a small resistor (say 47 ohms) kills ripple down quite a bit if used in conjunction with fairly large filter cap after it; I'd prefer a choke though, instead of a plain old resistor.

I know--ripple (mostly) cancels in a balanced stage if tubes are reasonably well matched. In any case it will be less of an issue if EFB is used, since EFB will track the voltage sag and auto adjust the bias accordingly. Thought I heard Wharf mention maybe using EFB on this amp if things work out...

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If WC sticks to the parts list specs there's 140mA to use. No sweat for the rectifier. If 30uF > choke 56R > 20uF = B+ to handle a 70W amp we all know and love, then just give me a dozen ea. 33uF, C-354, 47uf caps for any dozen EL-84 amps.

Maybe I'm just a simpleton but I have no idea what your point is. And what's a C-354?
 
If you want less sag in a PS , a Choke from what gadget73 and a few others are saying you want one with with lower resistance , like close to 20 ohms, and Edcor has them for reasonable $.
 
OK, good thoughts here on the choke. But, as indicated about the ST-70 and the C-354 choke, I believe those Dynaco folks figured that the DC power supply was OK with this choke in line. The Hammond 156R has nearly the same specs as the C-354 with the same 50 ish ohms of DC resistance, and 1.5 H induction. The 354 is I believe rated at roughly the same 200 ma of current as the 156. So, I'm thinking that since I have a 156 here, that's what I'll use. Also, I might add that the OTs in this build are perhaps a bit less demanding than those in other EL84 builds.....and certainly less so than those in the ST-70. As this amp isn't intended to be anything other than a 'functional daily drive / work horse' type unit.... I think that 156 will be fine. There's still a chance that I'll need a dropping resistor some place in the PS as well. So, as all this is still a 'work in process'.... I think I'll have to do some 'punting' down the road. WC
 
A pair of EL84 at full scream at typical datasheet conditions is good for about 115ma of plate + screen current, so 230ma for a stereo amp. Add a few for the driver and you're at ~235ma at full output power. Idle you're at 160ma for the output plus a few for the driver, call it 165 and its likely not far off.

Likely that EFB can reduce that though. Both of my EL84-type amps with EFB run at fairly low current. The one seems happiest at 23ma per-pair idle current. The Magnavox I think runs 44ma per-pair. Not a clue what they do at full output, I don't believe I ever measured it.
 
Thain, I took a rather different approach to what I thought might end up being the total current level in this amp. I simply took a look at an ST-35 schematic, noted the cathode resistor in that amp was a 95 ohm value, then looked at the voltage across it....which was listed as being 13.5 V. I simply did the math to determine that this was about 142 ma of idle current draw on the outputs. Like you said, adding a few more ma for the driver tubes, and I put the thing at still being under 150ma of total current. I then looked at the PA-774 power transformer itself and it is only rated at 180ma of DC current 'rectified'. As you noted, something like EFB might actually reduce this total current even more, particularly at full output. But, again, the OTs in the amp I'm building are only rated at 15 W each, so I believe that running the EL84s at full tilt is somewhat out of the realm of possibility when using these output transformers. If I understand it correctly, they're simply not up to that level of current draw. So I figure the output section in this project is going to have to be somewhat 'under-built' in order to use these OTs in conjunction with the EL-84 tubes. In so doing, I don't think I'll have any problem using that 156 Hammond choke. I've seen plenty of old amps running at about 12 Watts / side......and they sound damn good! That's all I'm hoping for with this. BTW, I also found a choke that I'd used before, a TRiad C-24X....which is 1H at 240ma and 50 ohms. I just ordered 3 of them from Arrow....... all 3 incl. shipping came to under $30. They too work well......and now I'll probably use one in this build.
 
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6, I have a couple of those, as well as a couple more on order. For some reason I believe I recall buying a pair as they related to a set of Heathkit amps I'd planned to rebuild. On a sadder note, those amps came in a 'swap' deal with our dear departed AK member AudioDon (RIP). I do plan to get to those one day!! Anyway, I recall that rectifier is a 'directly heated' unit like the 5U4, right? So, not quite as beneficial in the 'slow-start' category as the 5AR4. But, about a 67 volt drop in that tube IIRC, similar to the 5U4, but with a 1A reduction in heater current. Not a bad consideration if needed. I suppose it's a lot better than another resistor in the PS circuit.
 
Hey, not to belabor this 'current' thing, but is not the Maggie 9300 series of amps an EL-84 PP unit? IIRC, that unit used a 6CA4 rectifier in it......which is only good for like 150 ma isn't it? Again, just thinking 'out loud' (so to speak.....or 'type'...). But, I'm thinking that this build here will be closer in PS related specs to that 9300 than the ST-35, just due to the lack of OT potential. Am I off base with that?
 
I did a 'Google' search on the tube, found some info on the DuncanAmp's page that said it had a 67 Volt drop. That seems a bit high.......but I guess the only real answer will be to complete a rough-up of the circuit and drop the tube in and see what happens.
 
You can pick the rectifier to use based on output power you want and slow startup characteristics you want, because if you add EFB, power supply sag really won't be an issue either way.

67V drop is about right for a 5R4, in my experience, at the DC current your amp will draw.
 
I won't commit to EFB in this amp at this point, mostly because I just don't know if the end product will merit the added effort and expense. But, should it end up that I have the room, time, and parts available, then I might go with it. I think that's a decision for after it's up and working.
 
9300 series of amps an EL-84 PP unit? IIRC, that unit used a 6CA4 rectifier

5U4GB.

I still suspect that if you're using the CM's power transformer, the voltages should come out awful darn close to what the schematic says they will be. You'll be down a few tubes from the tone amp section but if all that adds to 10ma I'd be surprised.
 
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