5E5-A low output volume

[Corcia]

I've built a tweed pro 5E5-A amp from a Mojotone Kit, this is not my first build: i've already built a couple of Deluxe Reveb, but i'm definetly not an amp technician.
The Amp seems to be working fine:tone is great, no hum, all controls are fine over the entire range, tha amp starts breaking up at 6-6,5 but...the output volume is not what i expected, i've compared with the Deluxe reverb (a kit from mojo i built) and the the volume of the pro seems to be close (lower!) to the volume of the Deluxe reverb.
the test has been done (obviously) using the same speaker (15" electrovoice EV15-L) with all the pots turned halfway and with the same guitar.
i've never tried another tweed pro before so i'm not able to tell if this behaviour is normal or not but i have strong doubts that an "almost 40W" amp could be as loud as a 22W amp.
btw: tubes on the 5E5-A are: 12AY7 GE NOS, 2 X 12AX7 Tungsol, 2 X 6L6GC STR Tube Amp Doctor, 5U4GB electro harmonix.
thank you All!

[Dave H]

It's cathode biassed and the B+ is only 395V. I'd guess it to be closer to 25W than 40W.

[Corcia]

It's cathode biassed and the B+ is only 395V. I'd guess it to be closer to 25W than 40W.

so you think it might be "normal" that it has a lower volume than a deluxe reverb? do you think there might be a way to improve output?

[loudthud]

The 5E5 was cathode biased with a 5E3 tone stack. The 5E5A was fixed biased with the "E-Series" tone stack. Both were specified to have 6L6GB output tubes, that's the earlier version with a 19 Watt plate dissipation rating. There are no transformer numbers or specifications other than B+ voltages. One difference to note is that the 5E5 has a 2500 ohm resistor to drop the screen voltage while the 5E5A has a 10K. I think Mojo only claims the amp is 20W. You certainly can't make any judgement by the positions of the controls, you have to actually measure the power output.

Does the amp have adjustable bias? What is the bias current in the output stage?

[Corcia]

Mojo declares its kit to be a "20-30W", amp has fixed bias, i haven't measured the bias current yet, i will do It using a male/female socket probe. Btw, which should be a proper bias current? Is there any Way to install a pot to adjust bias?

[Dave H]

Sorry, I didn’t notice it was a 5E5-A. I thought it was a 5E5. I was going to suggest changing it to fixed bias and an SS diode rectifier but it’s already fixed bias.

I did build a similar circuit with a B+ of 375V and cathode biased it was only about 18W and not much more (20W?) fixed bias. The primary impedance was 8k though as it was a converted 5E3. A different fixed bias amplifier I have with a 4k primary and a B+ of 440V measures about 30W with 5881s and 35W with EL34s. It was biased at 35mA with 5881s and 40mA with EL34s.

It should be possible to make the bias adjustable. Can you post a schematic of the circuit?

They claim here that the 'correct' OT primary impedance is 6.4k.

[Corcia]

Sorry, I didn’t notice it was a 5E5-A. I thought it was a 5E5. I was going to suggest changing it to fixed bias and an SS diode rectifier but it’s already fixed bias.

I did build a similar circuit with a B+ of 375V and cathode biased it was only about 18W and not much more (20W?) fixed bias. The primary impedance was 8k though as it was a converted 5E3. A different fixed bias amplifier I have with a 4k primary and a B+ of 440V measures about 30W with 5881s and 35W with EL34s. It was biased at 35mA with 5881s and 40mA with EL34s.

It should be possible to make the bias adjustable. Can you post a schematic of the circuit?

They claim here that the 'correct' OT primary impedance is 6.4k.

The schematics and the layout i've used are available here: Mojotone Tweed Pro Style Amplifier Kit - Mojotone.com , in order to check voltages i referred to the standard fender schematics here http://ampwares.com/schematics/pro_5e5a.pdf
I've noticed another strange behaviour of the amp as a sort of correlation of the two volumes: if the guitar is in the bright input turning the normal channel volume to max makes the amp sound luoder, fatter and brings some hum in, when guitar is plugged in normal channel, turning up to max the bright volume makes the amp sound louder,thinner and brings some hum in.
I've checked voltage and there are few spots where voltages are not the same as in fender schematics:
-12ay7 cathode is 2.2/2.4V instead of 1.9V
- 12ax7 (pre) 1st cathode is 1.46V instead of 1.9V
- 12ax7 (pre) 2nd cathode is 170V instead of 130v
- voltage at 1n4007 diode (bias?) is -43V instead of -32V

B+ is 400V, using a SS rectifier it rises up to 434V
Primary impedance of the OT provided with the kit is 6k

[Dave H]

...I've noticed another strange behaviour of the amp as a sort of correlation of the two volumes: if the guitar is in the bright input turning the normal channel volume to max makes the amp sound luoder, fatter and brings some hum in, when guitar is plugged in normal channel, turning up to max the bright volume makes the amp sound louder,thinner and brings some hum in.

That’s strange. The 5E5-A uses a ‘virtual earth’ style mixer to combine the channels so I wouldn’t expect there to be much interaction between them. I’d double check your vol pots and input socket wiring. It could be something daft like the channels being crossed over at the inputs.

The bias current could be fine as it is but I’d measure it to make sure.

[g1]

- voltage at 1n4007 diode (bias?) is -43V instead of -32V

That is quite a big difference. Sounds like it is biased very cold. Check idle current of the output tubes.

[Corcia]

That is quite a big difference. Sounds like it is biased very cold. Check idle current of the output tubes.

G-one you are definetly right! I've checked idle current is 26,4mA, it should be @ 52mA for 6L6GC correct? Just to be sure: idle current is the current in pin 8 of 6l6?

[Corcia]

I would like to make the bias adjustable, i would add a trimpot in place of one of the two resistors adjacent to the diode, but which resistor should i replace? The 56k one or the 10k one?

[pdf64]

I would replace the 56k with 33k in series with a 47k trimmer.
Pete

[loudthud]

Something like this should work. If you can't get the bias current high enough, try a lower value resistor for the 33K, perhaps 27K or 22K.

[Corcia]

Way not Just a 56k pot? Connecting only one Pin and wiper

[loudthud]

You don't want to allow the bias voltage to be adjusted to zero. That could damage the tubes or the output transformer and possibly start a fire.