5E3 home build power dropping

[Jewellworks]

I should also mention I had a 12AX7 in V1 when I measured voltages and I'm sure that was adding a lot more fuzz , and I don't have a AY7, so I've got an AU7 in there now. Still buzzy, but at lower volume?
O scope tomorrow

 

[dan40]

Does the bulb test good? Huh? I thought the bulb was in series with the amplifier?

Sorry...brain fart on that one.

 

[printer2]

Sorry...brain fart on that one.

We all get them.

 

[Jewellworks]

EUREKA!
I got to thinking that maybe the owner got the wrong OT when he was sourcing parts, so I finally did a Windings Ratio check on it, and it's 50:1, 2500 imp rat, and at 8k load (2, 6V6s in PP), it needs a 3.2 speaker. The best i can do is a 4. He had an 8 on it.
But more importantly, I discovered I still had the OT wired wrong! I fixed it right this time, and now it sounds MUCH BETTER that it has, by FAR. No fuzzy buzzy, but it can still get like that around 7 or 8... But I think it's because I measured voltages and the PD is 25 & 26W each 6V6, so that's way stupid hot...
Need to raise the shared bias resistor, cool those tubes down, and this thing might start sounding like a good guitar amp.

 

[Jewellworks]

So now I'm completely confused...

I have my buddy's 5E3 here to compare it to the other 5E3 I've been working on, in this thread. After all the other problems I've fixed, I was suspecting the tubes were weak ir something else... So I measured the voltages in his known good, working amp, and then moved the tubes to this guys amp, and compared.
1). I'm getting the same results as the other tubes. So it ain't the tubes. Im suspecting the shared bypass cap? It's rated at 25v and it idles over 30.

2). Let's talk about these voltages...
My buddy's amp has the 250ohm shared cathode resistor. 19.6v. that's 78.4mA, and the plate is at 371, for a PD of 29W! Max is 9! Even on robrobs "5E3 typical voltages" chart, he has [email protected]=80mA, 362v=28.9W. so it's supposed to be THAT HIGH?

I've been trying to get the voltage and PD down and struggling. Am I chasing something to nowhere?

 

[Lowerleftcoast]

My buddy's amp has the 250ohm shared cathode resistor. 19.6v. that's 78.4mA, and the plate is at 371, for a PD of 29W! Max is 9! Even on robrobs "5E3 typical voltages" chart, he has [email protected]=80mA, 362v=28.9W. so it's supposed to be THAT HIGH?

Your math is not accurate.
78.4 / (2 tubes) = 39.2
39.2 x 351 (plate to cathode voltage) = 13.8W

 

[Lowerleftcoast]

Max is 9!

The cathode biased amps run at ~100%MPD not 70%

 

[printer2]

The cathode biased amps run at ~100%MPD not 70%

Why? As long as the sound is pleasing the greater the current the shorter the tube's life.

 

[Lowerleftcoast]

Why? As long as the sound is pleasing the greater the current the shorter their life.

True but the amp that was measured displays 13.8W. The OP was worried about taxing the tubes at 29W. Imo, steps taken to reduce to 9W are not necessary. A typical 5E3 would need some very cool tubes to run at 70%MPD. It is much more likely the circuit will end up around 100%MPD.

 

[Jewellworks]

Your math is not accurate.
78.4 / (2 tubes) = 39.2
39.2 x 351 (plate to cathode voltage) = 13.8W

You da man! I knew I was off on something. 2 tubes. Shared cathode. Half the mA each. That makes a lot more sense. But 13.8w is still too high. I put a 620 instead o the 250 and it's ideling right at 8.6w. comfortably under the Max of 9.
I also bumped up the bypass cap from a 25v to a 50v, since it's bias is at 31ish volts, and not 20 w the 620 resistor.
With a 12AU7 in the front, it stays cleaner longer.
Thanx guys. That was it.

 

[Lowerleftcoast]

I don't know why you are hung up on the 9W figure. Fixed biased amps need to idle below something near the 70%MPD because they will make it past 100%MPD when driven with a strong signal. Cathode biased amps, when driven with signal, will reduce from whatever idle %MPD is chosen. That is why they can idle at 100%MPD.

If you are happy with the sound, I guess that is all that is important.

Carry on.

 

[Jewellworks]

I saw the 9W in the PP, A/B column and thought that was it. Here it's 12W w cathode bias. I'm adjusting closer to 11W. I put a 470 in there and it's at 10W. 330 ought to do it .
But you're the one that reminded me the mA was half the value per tube. That was the key to my problems.

 

[Lowerleftcoast]

Just don't change your buddy's amp. He might not approve of the change.

 

[Jewellworks]

One thing I want to do before I change anything else is to use my Variac and set it to 110v, and then do all the measurements and see what the bias and PD is. Wall voltage has gone up 10v since the 50s, and that usually means about 30 volts on the B+, shifting everything else. I'm curious as to where this was idling in the 50s compared to now.

What raises my eyebrows is at 120v mains, we're idling at 12.3W. where was it when this amp was designed? -Not maxed out I'm guessing...

BUT... Theory now... Let's say in the future the wall voltage goes up even more. 150v... Now all these amps are going to be having real problems. The tubes can't take 400+volts. So some other means of dropping the voltage will be required. Probably a Variac because the filament voltage is also going up when the mains goes up. I know Bob Gjika voids the warranty if you don't run his amps on a Variac. He's got test points on the filaments and you set your Variac to where it says exactly 6 3v. -he ain't wrong...

I want to thank everyone for all your help and input on this. All my amps have been SE, and I'm not too experienced w PP nor PI circuits. -Slightly different animal... Other considerations and things. Thanx again

 

[Lowerleftcoast]

Imo you are assuming things that are not necessarily true. Back in the day we called it 110V (I still do) but in many (most?) places it was more.

To illustrate my point, Fender introduced the 5F6-A Bassman in 1958. The 5E3 was introduced in '57. Note the wall voltage on the 5F6A schematic below. (The Twin Amp 5F8A schematic shows the same.)



While you look at the 117V on the schematic, don't shy away from the *notice* in the top right corner *+ or - 20%*.

Now, I won't argue the ideal heater voltage for many a tube is 6.3V but a few winds variation on a transformer will result in more or less than that ideal. Many tubes have a heater voltage tolerance of +10%, -5% so we have a fudge factor for the tubes and also for the amp. The tolerance on the resistors were 5 and 10%. Capacitor tolerances can be up to 50% so don't get too hung up on exact values. What are the tolerances for the transformers??? Iow, try not to be the guy that uses 1% resistors on a build unless you have blueprinted a GOAT amp and are trying to make a perfect clone.

An economical way to reduce voltage is to use a *Bucking Transformer*. (There are many other names for this device.) A Variac is a useful tool, but I wouldn't want to carry one around to every gig.

 

[Jewellworks]

Imo you are assuming things that are not necessarily true. Back in the day we called it 110V (I still do) but in many (most?) places it was more.

To illustrate my point, Fender introduced the 5F6-A Bassman in 1958. The 5E3 was introduced in '57. Note the wall voltage on the 5F6A schematic below. (The Twin Amp 5F8A schematic shows the same.)

View attachment 1087063

While you look at the 117V on the schematic, don't shy away from the *notice* in the top right corner *+ or - 20%*.

Now, I won't argue the ideal heater voltage for many a tube is 6.3V but a few winds variation on a transformer will result in more or less than that ideal. Many tubes have a heater voltage tolerance of +10%, -5% so we have a fudge factor for the tubes and also for the amp. The tolerance on the resistors were 5 and 10%. Capacitor tolerances can be up to 50% so don't get too hung up on exact values. What are the tolerances for the transformers??? Iow, try not to be the guy that uses 1% resistors on a build unless you have blueprinted a GOAT amp and are trying to make a perfect clone.

An economical way to reduce voltage is to use a *Bucking Transformer*. (There are many other names for this device.) A Variac is a useful tool, but I wouldn't want to carry one around to every gig.

Not arguing with you at all. As an experiment I did like i said above: set my Variac to 110 and measured voltages, did the math, and the PD was almost exactly 11w. However, the heaters were around 5.7vac. when I set the Variac to where the heaters read 6.3, the Variac was at 118v. So yeah. I'm making an assumption that may not be correct, but now I know.

I've seen posts here for bucking transformers. Robrob has a schematic or something, but never knew what it was exactly.

 

[Lowerleftcoast]

I've seen posts here for bucking transformers. Robrob has a schematic or something, but never knew what it was exactly.

Yep, Rob shows a bucking transformer design on his 5E3 Mods page.

 

[Lowerleftcoast]

For giggles, pop your friends 5Y3 rectifier tube in your amp and check the voltages.

The Stancor PT lists 315-0-315 at 70mA. I would expect a little lower B+ with that PT. Not all rectifiers drop the same amount of voltage. It is worth a try.