1966 Guild T1 RVT - REVERB not working

[TelRay]

Hi all,

this is a nice little amp with a particular (and new for me) reverb circuit.

I was helping a friend to re-cap it during the lockdown using videocalls and making shopping lists together on Mouser.

Somehow he's lost the REVERB in the process (it worked when he bought the amp) and now I have the amp with me and trying to bring it back to life.

Here is the SCHEMATIC

What's new for me is that the AUDIO SIGNAL that feeds the REVERB circuit is coming from one of the wires going from the OUTPUT TRANSFORMER to the MAIN SPEAKER and that the unit has a DEDICATED REVERB SPEAKER (smaller) which also plays the DRY signal when the REVERB is OFF.

I've been following the signal path in two ways:

- CIRCUIT as in the SCHEMATIC:

first there were a few wiring issues that I corrected
- the REVERB feed signal was being taken from the NEGATIVE SPKR wire (which is also connected to ground). So there was no signal. Actually the speaker connector to the circuit was inverted.
- The electrolytic capacitor polarity was replaced with a new one and the polarity was inverted

additionally:
- the value of the e-cap is not the 3 uF indicated on the schmatic (the closer we could get was 5 uF wich actually measures 6.1 uF). So the frequency is cut below 200 Hz instead of 400 Hz as intended originally (but I guess this is more a matter of taste than a potential functional problem)




Here I see that:
- the REVERB SIGNAL is lost after the REVERB TANK INPUT
- the DRY signal is lost after when trying to come out of the V6 tube Pin 6

- Feeding signal to the REVERB TANK OUT

to check the rest of the circuit after the REVERB TANK



the schematic is missing any reference voltages other than the HV right after all the big capacitors so I am not sure of what voltages to expect at the REVERB TANK (I guess some is needed to drive the transducers unless they are driven only by the magnet) or at the 6BM8 V6 tube.

6BM8 V6 tube VOLTAGES:

PIN 1: 0 VDC
PIN 2: +60VDC
PIN 3: 0 VDC
PIN 6: +340 VDC
PIN 7: +340 VDC
PIN 8: +1.6 VDC
PIN 9: +146 VDC



any ideas of how to continue troubleshooting this one?

thanks!

[The Dude]

Have you checked that transducers or connections to the tank are not open?

[g1]

What is the resistor at pin2 of 6BM8 measuring, and does it match it's colour code?

[TelRay]

Have you checked that transducers or connections to the tank are not open?

yepp... checked continuity between all GREEN DOTs with MMT and then the BLACK dots in the photo below, both transducers checked

[TelRay]

What is the resistor at pin2 of 6BM8 measuring, and does it match it's colour code?

it measured 335 Ohm (without taking it out of the circuit)

I need to re-check this area of the circuit because the ecap across this resistor is new, so potentially a problem there

[g1]

60V across 335R is around 180mA, which seems quite high.
Is it marked as a 330R ?

[Helmholtz]

60V across a 330R cathode resistor simply can't be. It would mean a plate dissipation of around 50W, OTOH a bias voltage of 60V would mean cut-off.
Sure, it's not 6V instead?

[g1]

60V across a 330R cathode resistor simply can't be. It would mean a plate dissipation of around 50W, OTOH a bias voltage of 60V would mean cut-off.
Sure, it's not 6V instead?

I calculated approx. 10W plate diss., not 50W ?
I am more inclined to cut-off though, as you say. Maybe cathode is not connecting right?

Edit: 10W is resistor diss., not plate. oops

[Helmholtz]

I calculated approx. 10W plate diss., not 50W ?
I am more inclined to cut-off though, as you say. Maybe cathode is not connecting right?

I was speaking about plate dissipation - not resistor dissipation.
Actaully cathode bias shouldn't be able to cause tube cut-off.

[TelRay]

hi guys, thank you for the comments

1) +60VDC at V6PIN 2

as I said, there's something odd going on around that 50 uF ecap that goes from PIN 2 to ground and has the 330 Ohm R across
I measured +60V directly on PIN 2, got 0 V at the lead of the resistor connected to PIN 2 and 15 V at the ecap. I know I should be getting the same voltages as al 3 points are connected and continuity test confirms they are. Additionally , that ecap is rated at 25V so I already know I shouldn't be getting 60 V.
I have disassembled this part of the circuit (it's an add-on with a separate board) and seen my friend placed a jumper because the board got damaged when replacing the ecap. So I will be investigating deeper.

2) OTHER HIGH VOLTAGE LINES

these are the voltage I am reading (numbers correspond to those in the schematic)
1. +365 VDC [Nominal 335 V]
2. +322 VDC [Nominal 300 V]
3. +271 VDC [Nominal 250 V]
4. +313 VDC [Nominal 290 V, but a bit hard to read on the schematic]

up to here voltages are below 10% higher than what's specified on the schematic (I am feeding 117 VAC to the circuit) however voltages 1 and 2 fluctuate +/- 5-7 V. This maybe normal as the rectifier is a tube (and not a solid state diode).
Voltages 3 through 6 do not fluctuate in more than +/- 1V

and now the two voltages that feed the REVERB circuit are too high +55-65% more though I've checked the 1.5K 10W, 1K 10W and 22K resistors and they are all OK

5. +370 VDC [Nominal 225 V]
6. +369 VDC [Nominal 205 V, but hard to read on the schematic]

but... there's another thing
the voltage coming out of the DIODE rectifying the negative voltage is -19V. I cannot read wht the schematic shows here.
and the two resistors (they seem old, I am not sure if original) are 10K and 4.7K when in the schematic (with a lot of effort) I want to read 68K and 33K
I see the ratio is about the same: 10K/4.7K (almost) = 68K/33K

In any case, I am concentrating my efforts on the REVERB circuit by now so, understanding what the hell is going on with VOLtAGES 5 and 6 s the priority

PS: all the big ecaps are new

60V across 335R is around 180mA, which seems quite high.Is it marked as a 330R ?

yessir... triple checked and measured now lifting the resistor as i am reworking the damaged circuit trace

[g1]

After the trace repair, with the tube out see if you measure 330R from pin2 of the socket to ground (check at tube side of socket pin).
And you should also read 330R across the 50uF in circuit.

[TelRay]

OOOKK... a lot of work done this evening: traces rebuilt, redid most of the new components solderings, changed some dubious looking wiring, etc.
and now... there is REVERB sound... very low in volume, but it's there

After the trace repair, with the tube out see if you measure 330R from pin2 of the socket to ground (check at tube side of socket pin).
And you should also read 330R across the 50uF in circuit.

yessir... there is now 331 Ohm from V6 PIN 2 to GND and also +15VDC. It seams that repair did some good

Now VOLTAGES 5 and 6 (feeding the REVERB circuit) are also what they should:

5. +206VDC [Nominal 225 V]
6. +192 VDC [Nominal 205 V, but hard to read on the schematic]

At V6 B6M8 I am getting

PIN 1: 0 VDC
PIN 2: +15VDC
PIN 3: 0 VDC
PIN 6: +193 VDC
PIN 7: +202 VDC
PIN 8: +1.0 VDC
PIN 9: +88 VDC

At V5 12AX7

PIN 1: +122 VDC
PIN 2: 0VDC
PIN 3: +1.0 VDC
PIN 6: +123 VDC
PIN 7: 0 VDC
PIN 8: +1.0 VDC
PIN 9: 0 VDC

As I read the cricuit the small REVERB SPEAKER should work like this:

- REVERB SWITCH ON: REVERB sound
- REVERB SWITCH OFF: DRY Guitar sound

What I am getting now is very low reverb sound with the switch ON and no sound at all (in this speaker, there is sound on the big one) in the OFF position

I am going to check the REVERB VOLUME potentiometer wirint (which I have replaced). Could I have inverted the wiring to lugs 1 and 3?

[Enzo]

OK, so th reverb speaker makes either really low reverb or really low dry. SInce you get no dry signal, I am going to assume the reverb control is not involved. It isn't even in the circuit with reverb OFF. SOmething in the reverb speaker amp is not right. COuld be anywhere from the switch itself to the speaker itself.

[TelRay]

OK, so th reverb speaker makes either really low reverb or really low dry. SInce you get no dry signal, I am going to assume the reverb control is not involved. It isn't even in the circuit with reverb OFF. SOmething in the reverb speaker amp is not right. COuld be anywhere from the switch itself to the speaker itself.

Enzo, your comment made me question what I hear so I hooked the scope right after the footswitch (marked with an "A" in the schematic). Concretely I put the probe between the 1K and 330K resistors because here I will see the signals coming both from the REVERB and DRY.



And... there is a signal from BOTH (wet and dry). I believe that with the low volume at the REVERB SPEAKER I could not tell when the DRY signal was coming out (because it is no different to the one coming out of the MAIN SPEAKER).

Here is the level of the REVERB signal at that point (remember my old scope is always tilted clockwise):



The DRY signal is a bit weaker... by about 10 mV (I'm counting 5 mV on top and another 5 at the bottom of the wave)

So I belive I do not have a problem up to this point, so there is an amplification issue after.

What I am thinking is to follow the signal and evaluate that every stage is incrementing the signal in the desired amount.

Is there any kind of a guideline to say something like:

- 1 mV input at 6MB8 pin 1
- 10 mV at 6MB8 pin 9
- 50 mV at 6MB8 pin 6
- 500 mV at the (+) REVERB SPEAKER terminal

thanx!

[g1]

See how much signal you can get at pin3 (control grid of pentode section) of 6BM8 with fairly hot settings of amp and reverb level. Should be able to get several volts I would think.