Guild thunder 1 help

[angelothewolf]

hi there, im working on this vintage guild thunder one, i have some diferences between the real circuit and the schematic.
first i replace some open plate resistors, and replace the can capacitors (40+40+40+40) with 47/450v caps. preamp and tremolo section works ok but i have some biasing issues with the output stage. and this 6GW8 arrangement is a little bit confusing.
All my voltages are like around 10v to 20v higher than the spec on the schem.

first thing, i have 100ohm 10w resistor with a 220uf parallel cap between cathodes and GND, where it should go straight to gnd according to the schem. secondly the junction of both 390k grid resistors are grounded too, where it should go to V1.

ive got like a 45ohm diference beween output transformer windings, 175 and 220ohm.
voltage applied and V3 start to get very hot and have a plate current reading of just a few mV. so grounding the cathodes dont seems to be a good idea.
So can someone give me an orientation here please?

Guild_Thunder_1_12.pdf

[52 Bill]

So can someone give me an orientation here please?

You are describing two entirely different types of power amps.

The schematic shows a fixed bias design, where the cathodes of the output tubes are directly grounded and a negative bias voltage is applied to the input grids.

Your amp is a self biased or cathode bias design, where the input grids are kept negative in relation to the cathodes by raising the cathode to a positive voltage level with the 100 ohm resistor.

Maybe you can try and find a schematic that better matches your amp.

[angelothewolf]

You are describing two entirely different types of power amps.

The schematic shows a fixed bias design, where the cathodes of the output tubes are directly grounded and a negative bias voltage is applied to the input grids.

Your amp is a self biased or cathode bias design, where the input grids are kept negative in relation to the cathodes by raising the cathode to a positive voltage level with the 100 ohm resistor.

Maybe you can try and find a schematic that better matches your amp.

well i think the same when i saw the schem but the model number of the amp matchs exactly with the number on the schematic (T-1-12) and that 100ohm resistor apears to be a later modification... all the schematics i found of this guild amps are fixed bias...

[52 Bill]

...and that 100ohm resistor apears to be a later modification... all the schematics i found of this guild amps are fixed bias...

Then you can put the amp back to the original fixed bias configuration by following the schematics.

[Enzo]

The output winding resistance is not an issue, many transformers are like that due to the way they are wound.

If your schematic matches your amp other than the bias system, note that grounding the 390k grid resistors will prevent the tremolo from working. perhaps someone rewired the amp for cathode bias and decided they didn't need the trem.

Your B+ voltages may all be higher, and that is quite normal, especially for amps that are 50 years old. AC mains voltages are usually higher now than they were then, so all amp internal voltages will be as well. Remember, this is just a guitar amp, not a precision piece of laboratory gear. VOltages and other values only need to be close.

If one of your power tubes is getting real hot, then swap places with the two tubes and see if the problem travels with the tube or if it stays with the socket.

[angelothewolf]

ok, i think i know whats going on here. The amplifier seems to be grid bias by the schem, but i have cathode resistor and i just have two windings on the power transformer(heaters & HT), so i think someone replace the power transformer, and make the amp cathode bias.

now, correct me if i'm wrong:
i measure 172ohm from op tranny ct to plate 1, with a voltage drop of 2V i got 11mA of plate current right? so with a plate voltage of 314V this givesme around 3,45W of plate disipation. the same calculation gives me 2,87W for the other tube.
so apears to be too cold bias but the cathode resistor seems pretty low to me (130ohm) what you think?

[g1]

What is the voltage across the cathode resistor? From that you can calculate the current through the 2 tubes and see if it agrees with your other calculation.

[angelothewolf]

What is the voltage across the cathode resistor? From that you can calculate the current through the 2 tubes and see if it agrees with your other calculation.

i got 8.3v across cathode resistor, so thats gives me the opposite. a hot bias. so what im doing wrong here

[g1]

The current through the cathode includes screen current, but that should not add so much. I would the cathode voltage reading more than the OT voltage readings.
Have you tried re-checking the OT resistance readings with the amp hot?

[angelothewolf]

oh my god! the cathode resistor drop resistance when it gets hot, i never seen that in my life

replace it with a 200ohm resistor and everything estabilized!

measuring by the cathode voltage drop i get around 8W of plate dissipation, gonna test it a little bit and see how it goes!

[g1]

I've never seen that either! Is there a cap in parallel with the resistor?
I guess if the 200 ohm cured it, you are good.
Maybe you could go through your OT voltage & resistance measurements again and see if they now line up with the cathode method, just for future reference.

[angelothewolf]

yep, but is not the cap. i take the resistor out and heated up with my rework station, then it start to rise the resistance, when it gets to 70 degrees celcius, it short out! incredible... thats why my measurements were all wrong now i get just a little more current with the cathode metod but it match with the OT measurement