Valvestate 8080 with quiet Boost channel

[Stratfordade]

I'm working on a Marshall 8080 where the power amp and clean channel work fine and loud, but the Boost channel is relatively quiet. Selecting the higher OD2 level of boost does boost the signal about 10x so that at least is correctly switching in IC1A for OD2 on attached schematic.

On first inspection I found several resistors series together to replace one of the 180 ohm heater resistors and the strain had lifted the pad, so I replaced these with a single 180 ohm 10W and repaired lifted pad and at least had heater voltage and some boost volume where they'd been almost none before.

The boost channel uses a double triode and on the anode of the first stage with valve in place I see 109V, and 186V on the anode of the second stage. Are those voltages correct? I read 281V at the cathode of the rectifying diode D3 from the auto transformer.

At the grid of the first triode R69 I see about 200mV but at the cathode of the first stage triode junction R70 I read only 50mV on the scope which doesn't sound like much of a boost! At the cathode of the second stage at top of R71 I read just a few volts with 10mV fed into amp input and the boost gain control set over half way. Does this sound right?

I'm pretty sure the problem is somewhere in this ECC83/12AX7 stage but it's getting HT. If that HT is too low where should I look for the cause? I did replace the ECC83/12AX7 with another one which I know is a working one from trying it in another amp, but it made no difference to the low boost volume or the signal being actually reduced between the grid and cathode of the first triode.

Thanks for any advice anyone can give me on this.

[Helmholtz]

What are the cathode voltages?
What is the heater voltage measured across tube socket pin?

[g1]

Your plate voltages are correct for this model. There have been a few repairs on them recently so I have those numbers temporarily available for recall.

[g1]

At the grid of the first triode R69 I see about 200mV but at the cathode of the first stage triode junction R70 I read only 50mV on the scope which doesn't sound like much of a boost!

Measure at the plate (R109) instead of the cathode.

[Helmholtz]

As cathode voltage follows grid voltage I'd expect the cathode signal to be at least 80% of the grid signal.
(except if the wrong grid pin was scoped ).

[Stratfordade]

Helmholtz doh should have checked that! The voltage across both heaters when warmed up is 13.7 so that's OK. However cathode voltage of the first triode is only 0.7V! The other triode has 118V at cathode.

@g1 measured at R109 anode 1 has 112V, anode 2 has 186V

Helmholtz I've double-checked I'm measuring the correct grid and cathode and when I have a 0.2V signal on grid 1 I only see 50mV on cathode 1.

[Helmholtz]

Helmholtz doh should have checked that! The voltage across both heaters when warmed up is 13.7 so that's OK. However cathode voltage of the first triode is only 0.7V! The other triode has 118V at cathode.

@g1 measured at R109 anode 1 has 112V, anode 2 has 186V

Voltages look good.
First triode has 0.7mA plate current, second one (CF) has 1.2mA.
Should work.

[Stratfordade]

Thanks Helmholtz that narrows it down. I'll look elsewhere for where I'm losing volume for the OD1 signal.

It definitely gets a lot louder when I insert the tube once it warms up, but scoping the signal at first grid I can't get it to clip in OD1 even with gain at max, whereas it will easily clip with the larger signal on OD2 setting. I can only get it to clip on OD1 with boost gain at max and input wound up to 30mV p-to-p which seems pretty high? Things would be louder if IC1A was boosting the overdrive signal more, so I'll keep looking around there and if nothing obvious shows up replace with a new TL072.

[g1]

@g1 measured at R109 anode 1 has 112V, anode 2 has 186V

I meant your AC signal voltage. You are looking for gain at the 1st triode cathode but you should be looking at the plate.

[Stratfordade]

I meant your AC signal voltage. You are looking for gain at the 1st triode cathode but you should be looking at the plate.

Ah forgive me silly of me. With 30mV at the grid of 1st triode there's 1500mV at the plate so a healthy gain! I have to wind the input up so there's almost a 1V signal at grid before it starts to clip with gain set at max. I'll look at replacing the IC1 op amp

[Stratfordade]

I think I can see what's going on. The Normal channel gain has about 50mV output at first op-amp with 10mV input which I think fits with the 100K and 22K resistors around IC1B. The Boost op-amp has 100K and 4K7 resistors in same positions so should have a gain of around 20, not the 10 I'm seeing. I'll check the resistors but could well be fault in the IC1 op-amp reducing the gain to half what it should be.

[Helmholtz]

Full schematic?

[Stratfordade]

Replacing the op-amp (full schematic attached) IC1 has definitely improved things. Before I could only get OD1 to clip at first triode with about 50mV at amp input with max boost gain, now at same gain setting it will start to clip with 25mV input.

[Stratfordade]

I'm pretty happy now with volume of the Boost channel played through the Celestion in its cabinet. The speaker itself seems to break up with anything bassy (I've checked with a very similar speaker and it's a speaker problem) so I'll look for a replacement for the G12-T that's in there as seems to have had some damage on the amp's journey to the thrift store where I found it!