I've just received an amp that (long story short) isn't working after shipment; no sound out except thermal noise from the power tubes. Trying to figure it out; it's a Marshall type amp.
Done a bunch of measurements and one thing struck at me. For two of the preamp cathodes, they have DC voltages at pin 8 of 109 and 127.
Can you possibly think of any reason why this would be correct? Even for a cathode follower tube?
Cathode Voltage - High?
Moderators: pompeiisneaks, Colossal
Re: Cathode Voltage - High?
In a Marshall amp (or any other variation on a Tweed Fender) there will be one cathode follower with high voltage on the cathode (though 127VDC seems a bit LOW actually). Unless there are two cathode follower circuits, I can't imagine why two cathodes would be high.
Some of the newer spiral filament tubes can't handle high cathode to heater voltages. Maybe a preamp shorted?
Some of the newer spiral filament tubes can't handle high cathode to heater voltages. Maybe a preamp shorted?
Re: Cathode Voltage - High?
Ooops, double post.
Last edited by gearhead on Sat May 17, 2008 3:19 am, edited 1 time in total.
Re: Cathode Voltage - High?
There quite possibly are two cathode follower circuits. This one is a unique one, built a few years back before I was doing amp stuff. Had sent it to the builder for a repair I couldn't diagnose and some bias mode mods (each tube separate).
It's basically two Marshalls in one box. There are two sections, each with it's own row of MV, T/M/B, Bright Vol, Normal Vol (Internally jumpered) knobs and two dedicated preamp tubes. Diff between the two is one has Bright/Normal in parallel, the other has them in serial, plus a little difference in voicing. Front panel or foot switchable between the two sections.
Needless to say, this is quite a FUN amp
When working.
If that is low for a non-PI cathode follower, that tells me something. Most of the other cathodes are also running lower than I'm used to; in the 0.6-1.2 VDC range.
It's basically two Marshalls in one box. There are two sections, each with it's own row of MV, T/M/B, Bright Vol, Normal Vol (Internally jumpered) knobs and two dedicated preamp tubes. Diff between the two is one has Bright/Normal in parallel, the other has them in serial, plus a little difference in voicing. Front panel or foot switchable between the two sections.
Needless to say, this is quite a FUN amp
When working. If that is low for a non-PI cathode follower, that tells me something. Most of the other cathodes are also running lower than I'm used to; in the 0.6-1.2 VDC range.
Re: Cathode Voltage - High?
Maybe all the voltages are on the low side on purpose. In most a cathode follower designs, the plate and the cathode will be at close to the same voltage. You could check that. In a typical common cathode preamp, the cathodes tend to be around 1.2 - 1.5, but might be lower if a small resistor is used. It will depend on the plate voltage of the stage and the type of tube, etc. etc.
It does sound like a fun amp.
It does sound like a fun amp.
Basic.
Was the amp a new amp?
If it is, I'd be looking for something real basic first. Maybe you have already checked out the simple stuff, remove all the tubes and replace them, pin conections sometimes do resistances.
Sorry but we (us humans) do jump in the deep end, I'm guilty too.
Anyways, good luck with it.
If it is, I'd be looking for something real basic first. Maybe you have already checked out the simple stuff, remove all the tubes and replace them, pin conections sometimes do resistances.
Sorry but we (us humans) do jump in the deep end, I'm guilty too.
Anyways, good luck with it.
Wheres the fire extinguisher John?
Re: Cathode Voltage - High?
No, it's not new. Was built about 4 years ago.
I -think- I found the issue, and am going to send it back to the builder. Don't have the parts ifin I'm right. Believe it's one of the relays.
Input sig gen (old Eico!). Got signal all the way to the tone stacks. Since they are separate, each is fed into a relay. No signal at the input to the PI, except a very low level, perfect, sine wave. Still learning to use my OScope, so not sure if it's self-induced or from the relay. In any case, it does NOT look like the input.
Thanks all!
I -think- I found the issue, and am going to send it back to the builder. Don't have the parts ifin I'm right. Believe it's one of the relays.
Input sig gen (old Eico!). Got signal all the way to the tone stacks. Since they are separate, each is fed into a relay. No signal at the input to the PI, except a very low level, perfect, sine wave. Still learning to use my OScope, so not sure if it's self-induced or from the relay. In any case, it does NOT look like the input.
Thanks all!