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Old July 2nd, 2013, 03:27 PM   #1 (permalink)
TNO
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5e3 voltages off

I have a 5E3 that sounds too clean. It's a Weber SoWatt/Mable build. I upgraded the PT to a Magnetic Components DR OT that has the correct voltages for a 5E3. With a RCA 5Y3 I'm getting 42ma, with a 5Y3 copper cap 40ma. Voltages are:
1st filter- 372v
6V6 pin4- 321
V2 pin6- 207
V2 pin8- 45.7
V2 pin1- 168
V2 pin3- 1.27

V1 pin1- 149
V1 pin6- 146
V1 pin8- 1.5

The V1 voltages are with a 5751 installed.

I would appreciate any guidance on what might be going on.

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Old July 2nd, 2013, 04:18 PM   #2 (permalink)
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What the reading at the cathode cap? Are you subtracting pin 3 from pin 8 of the power tubes to get volts on plates?
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Old July 2nd, 2013, 06:32 PM   #3 (permalink)
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I will measure pins 3 and 8 and get back to you. Could you be more specific about the cathode cap?
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Old July 2nd, 2013, 06:48 PM   #4 (permalink)
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Pin 8- 360
Pin 3- 20
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Old July 2nd, 2013, 07:18 PM   #5 (permalink)
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TNO, ime, it is better to give voltages for one tube and then separate that info from the next tube. Less confusion.
V1---12_-7
...pin 1
...pin 3
...pin 6
...pin 8

V2---repeat

V3(6V6)
pin 3---plate
pin 8---cathode

V4...6V6...repeat

We would like to know the value of the bias resistor, also. The current draw measurements that you gave us are of interest. That reading should be taken with each rectifier...as should the tube voltages. Those voltages and the current draw measurements change with the change in the rectiifer.
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Old July 2nd, 2013, 07:41 PM   #6 (permalink)
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Thanks Wally, I'll pull all that together.
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Old July 2nd, 2013, 09:55 PM   #7 (permalink)
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RCA 5Y3

V1-5751
1- 153
3- 1.5
6- 150
8- 1.5

V2- 12ax7
1- 170
3- 1.27
6- 208
8- 46.5

V3- Tung Sol reissue 6v6
40.8mA
3- 370
8- 21.3

V4- Tung Sol Reissue 6V6
43.1mA
3- 368
8- 21.3

1st filter 33uF- 386
2nd filter 10uF- 329
3rd filter 10uF- 257

250ohm cathode resistor, unbypassed- 20.8
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Old July 3rd, 2013, 05:12 AM   #8 (permalink)
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All looks fine....what is the problem???
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Old July 3rd, 2013, 09:18 AM   #9 (permalink)
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For the power tubes, you have to subtract pin 8 from pin 3. This give you the real volts on the plates. Your numbers seem to indicate to me that you are right where you want to be. What issues are you having?

What speaker are you using?

What are the specs of the xformers? You said DR PT? Or DR OT?
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Old July 3rd, 2013, 11:09 AM   #10 (permalink)
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I used the Classictone DR PT that is lower than their tweed PT for some reason- 330-0-330. The amp is cleaner than the 5E3 I built and sold a while back. I did the brown deluxe volume/tone mod (with no tone control on channel one). Could just be that the mod dumps some gain? With a stock 5E3 turn one channel up to around ten and it's raging. With my amp I'd have to jumper both channels dimed to get there and, while that sounds great, it still won't do the Neil Young about-to-explode thing.
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Old July 3rd, 2013, 11:23 AM   #11 (permalink)
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I'm not familiar with the mod you mentioned , my guess is that the mod may have taken some of the snarl away . The tone control makes mine break up early if it's set above 6, and if I understand correctly you have taken that out of the normal channel? I'm not familiar with the preamp tube in V1, is it higher gain than the standard 12AY7 normally in that position? If it is lower gain tube that might also account for the additional headroom you are having, are you using a higher efficiency speaker than the usual suspects found in most 5E3s?
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Old July 3rd, 2013, 12:24 PM   #12 (permalink)
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My understanding is that no tone on the normal channel would boost gain. 5751 I think has about double the gain of a 12AY7? I probably should put it back to stock and see what it does for gain but the clarity of brown deluxe setup is nice. Trade offs...
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Old July 3rd, 2013, 12:39 PM   #13 (permalink)
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TNO, FWIW...just an observation or two....or 3...
First, the lack of a bypass cap on the bias circuit will cut gain.
Secondly, that tone control mod will suck gain. The second gain stage in the 6G3 is needed to compensate for that, I think. Without that second gain stage, that preamp signal might be weak.
Thirdly....daisy chaining those channels does not increase gain. Daisy chaining does not 'stack' gain stages but rather parallels them..no gain increase...just tonal complexity. I know...there is a lot of 'net hype about increasing gain this way, but it simply doesn't happen. Now, if your 'Normal' channel is closer to stock, that channel may be 'gainier' than the modded channel, and one might perceive an increase in gain when daisy chaining due to that. But...a comparison of the Normal to the Bright by themselves might reveal that the Normal channel is indeed closer to a stock 5E3 than the modded cahnnel when considering gain.
Lastly....Imho that current draw is a bit low for a cathode biased circuit like the 5E3. IT is perhaps okay...leaning to the cool side...for a fixed bias circuit, but cathode biasing can tolerate....and maybe needs...more current draw than that to get things going. Your tubes may be on the 'cold' side of things. That is, in a cathode biased circuit one biases by either changing the bias resitance or by choosing tubes that run in the desired range of current draw. The 70% of max plate dissipation does not apply in this circuit, imho and ime. A good sounding 5E3 will be closer to 100%. I once 'cooled' one down for a fellow who did not like the 'stock' type of sonic. We ended up with about 92% of max plate dissipation....12.9 watts. He now has to run the same 'grade' of tube to maintain that sonic.
MY thoughts are that you don't have a 5E3 there. You have a modification....sort of a bastard child of a 5E3 and a 6G3. I would suggest that you first install a bypass cap on the bias circuit to see what that does. IF that doesn't get you where you want to go, then take a look at your biasing/current draw/ plate dissipation and see what is going on. NOte: that current draw figure needs to be divided by 2, doesn't it, since the tubes share a cathode biasing resistor?
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Old July 3rd, 2013, 03:20 PM   #14 (permalink)
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Thanks Wally. What I'm going for with the lack of a bypass cap and the 30-10-10 filters is getting closer to a Gibson GA-6 I used to own. I tried adding the bypass cap back in and it raised gain but l lost some of the sweetness and compression. I'm running a REALLY clear speaker (Eminence Lil' Texas) and probably don't need the extra clarity that the vol/tone mod provides so I'm going to put the front end back to stock. That might be all I really need to do. I've owned a bunch of old Gibsons and a 5E3 clone and most of those amps were in the sweet zone set up with no bypass cap on the 6V6s and a 5751 in V1.

I think I see what you are saying about the current draw. What sent me here in the first place is that when I checked it with a bias probe it looked way high at 40-43mA. So what you are saying is that even though I'm measuring it at the tube socket I still divide by 2 since they share the cathode resistor? I'd like to leave off the cathode bypass cap on the 6V6s for now. To raise current draw would I lower the cathode resistor value slightly below 250ohms?
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Old July 3rd, 2013, 03:44 PM   #15 (permalink)
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Ohms law...more current less volts...more volts less current.
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Old July 3rd, 2013, 04:36 PM   #16 (permalink)
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Yes, TNO...when you measure that current draw at one tube in that circuit, you are looking at the combined current draw of both tubes.
Re: GA-6 versus the 5E3....quite different amps despite having the same tube compliment. If I were you , I would build the 5E3, and then I would build a GA-6. Or....rebuild what you have there but build it as a dedicated GA-6. Check out the PI situation.....among a few other differences.
I agree that the sonics of a good GA-6 from the '50's are a thing of wonder....but a 5E3 they are not....not better or worse...but different they are.

Yes, lowering the bias resistance will increase current draw...but there is more to it than that....much of it beyond my understanding. AS I noted, changing to tubes with 'hotter' operational specs will do the same thing.
Usuaslly,this is enough to bring one of these amps around. IF I were dealing with that amp, I would put it into a known arrangement before I started trying to get it to do something else. IF you have another pair of 6V6's lying around and when it it is time, sub them in and check out if there is any difference. IT would be interesting to have several 'grades' of matched and graded tubes just to mess with in this type of situation...it is informative, because just as in a fixed bias amp, different tubes act differently in circuit....unless they are all matched to each other. This is why I don't pay a lot of attention to things like ''these new *whatever brand* tubes sound so much better than this other brand'' unless I know what the biasing/current draw/plate dissipation figures are, right? A change in biasing or a change in tubes can either make an amp sound glorious or sound terrible in no time at all.
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Old July 3rd, 2013, 05:36 PM   #17 (permalink)
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I put the front end back to stock and it's gainier and greasier. I appreciate all the input.
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Old July 3rd, 2013, 09:59 PM   #18 (permalink)
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I scrounged up a 10W resistor that metered at 237ohms. Tried it in place of the 250ohm (which measured 247) and now the amp has more sparkle and shimmer, edges over into drive seamlessly- it just sounds great. I'm too beat to check voltages and current tonight but it seems to be a step in the right direction.

Also replaced the Lil' Texas with a Jensen Neo and the Neo is a much more well-rounded choice.
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