Old plexi Calsbro very low volume

[booba]

Hi all,

I am new here, and I joined because I love this forum, pretty competent and friendly techs here

I have an ooooold Carlsbro 60 "plexi" head. Pretty nice end of 60s / early 70s british stuff.
The one you can see here : http://i.ebayimg.com/t/VINTAGE-1968-...)C!~~60_57.JPG it's the same but with original multicolored knobs.
After opening it, according to Chambonino's site, the schematic for this amp is the CS60 BTC, because the two caps from v1 anodes are .047 mustards (otherwise almost the same as a CS60 TC)
Here's the schem : Carlsbro CS60 BTC Amp Schematic

Now, I've got a problem with it. The output is very low, my 10 watts combos are louder.
Everything seems to work fine, I have voltages everywhere, in the correct areas / ranges.
Power tubes
Anode / screen / grid : 460/ 458/ -36
Phase inverter 230v anodes, 30v cathode
preamp tubes 200v anodes (because I swapped the 100k plate loads for 10k resistors to get more dynamic range), 1.3-2v cathodes
Signal caps are not leaky. Fresh power supply and bias caps. Pops everywhere where it should when I test. The amp is ok, as the volume increases with the pot, the EQ is working, as is the "top cut" filter. Preamp tubes ok and tested. Tried with fresh powertubes also. In fact the amp works really well, but has no "Ooommppph" kicking in, no power at all.

I'm suspecting a bad OT.
From center HT to plate windings, I get 80 ohm on one side, 55 on the other. 133ohm plate to plate disconnected.
What is VERY strange in this OT are the secondaries. 9 wires !! 2 black common wires, one single green 16ohm, and two groups of 3 wires for 4 and 8 ohm taps.
There is proof inside the amp there has been output tubes problems, replaced screen resistors, resoldered OT primaries...someone already tested this OT.
Picture of the OT :

So...I would like you to tell me if I missed something. I do not have access to a voltage generator to test the secondaries (maybe a battery?). Any thoughts ?

Thank you very much !

[Jazz P Bass]

To verify whether or not the OT is the culprit you need to take a few Vac measurements with a signal applied.

Insert a 100 mv/ 1K signal into the preamp input.
With the controls full on, measure the output tube grid pin signal.

Then compare that to the OT signal into a proper dummy load.

If it is a 60 Watt amplifier, you should read a 22Vac signal at the dummy load (@8 ohms)

[booba]

Hi and thank you Jazz.
Sadly, I don't have access to a signal generator right now.
I'll try to locate one If I can't find any other way to find my problem.

What is strange to me are the multiple secondaries. maybe as it has been redone there is a bad wiring at the impedance selector.. . I'm pretty confused.

[Dave H]

...preamp tubes 200v anodes (because I swapped the 100k plate loads for 10k resistors to get more dynamic range), 1.3-2v cathodes

Well if you swapped all four that would knock the gain down by about 20dB on both channels which would definitely cause a lack of oomph. Does it distort when it's cranked up or is it always clean?

[booba]

Well if you swapped all four that would knock the gain down by about 20dB on both channels which would definitely cause a lack of oomph. Does it distort when it's cranked up or is it always clean?

It's always clean. If I strum a power chord very hard with a humbucker-equipped guitar i can hear a tiny polite beginning of crunch.
You're right, I will investigate there : didn't thought of that at all :-)

[olddawg]

There is a plethora of free signal generator apps that you can download for your phone or computer. I use SigGen.

[booba]

I rebuilt the preamp with the correct plate load resistors and indeed I get the gain (as in "crunch") back. Full tone.
But I'm still really low on volume, the initial concern. A big power chord gives me 3.9Vac through speaker terminals !

(Oddawg thanks for the tip. Will download it !)

[booba]

To verify whether or not the OT is the culprit you need to take a few Vac measurements with a signal applied.

Insert a 100 mv/ 1K signal into the preamp input.
With the controls full on, measure the output tube grid pin signal.

Then compare that to the OT signal into a proper dummy load.

If it is a 60 Watt amplifier, you should read a 22Vac signal at the dummy load (@8 ohms)

You mean an AC test on the grids, pin5 ? Reading pin5-to-pin5, I get a 108Vac signal, amp maxed out (with a guitar). And less than 4Vac on the speaker ends.

[Jazz P Bass]

You mean an AC test on the grids, pin5 ? Reading pin5-to-pin5, I get a 108Vac signal, amp maxed out (with a guitar). And less than 4Vac on the speaker ends.

If that is the case, then I would think that either the output tubes are bad or the OT is.

[booba]

Swapping in another EL34 pair, the max i can get is 6.0Vac at the speaker jack.

[Jazz P Bass]

If you have double checked the screen resistors as good and the ground connection to pins 8, then that leaves the OT.

[Ted]

I've seen several in the last few years. Ultrasonic oscillation cooks the O/P transformer. It then stops oscillating. Until you replace the transformer....

Fiddling with the lead dress sometimes fixes it. How old are the power supply electrolytics?

You really need to test that output transformer.
I test O/P transformers by putting a 1V 50Hz AC signal into the secondary and measuring the voltage on the primary.
You really can't tell very much by measuring dc resistances.

[booba]

If you have double checked the screen resistors as good and the ground connection to pins 8, then that leaves the OT.

Yes I checked them twice, and grounds are ok. :-(
Thank you so much jazz for your help, greatly appreciated.

[booba]

I've seen several in the last few years. Ultrasonic oscillation cooks the O/P transformer. It then stops oscillating. Until you replace the transformer....

Fiddling with the lead dress sometimes fixes it. How old are the power supply electrolytics?

You really need to test that output transformer.
I test O/P transformers by putting a 1V 50Hz AC signal into the secondary and measuring the voltage on the primary.
You really can't tell very much by measuring dc resistances.

Hi ted,
and thank you.
The electrolytics are brand new now, but were the originals when I bought the amp (and the volume prob was already here).
I have to find a way to inject AC voltage into the secondary. Time to find a low-AC source.
But anyway, as there is NO part of the amp that is faulty (it seems!!),that leaves me to the OT...but you're right the wiring is kinda so-so in this amp. I'll redo that and use shielded wire on strategic points.

Jazz : is 108Vac (on the grids of the output tubes)when signal applied a standard / acceptable post Pi-value?
If so that means everything BEFORE the output stage is ok?

[Jazz P Bass]

Jazz : is 108Vac (on the grids of the output tubes)when signal applied a standard / acceptable post Pi-value?
If so that means everything BEFORE the output stage is ok?

Without a scope it is hard to say that the signal is o/k, but the amplitude should drive those two EL34's to 60 watts, easily.