This amp sounds fine up until the grids of the output tubes. The output is weak and distorted. What's the problem here?
Plate Voltages 412V
Screen grids 388V
The third node 363V
Bias -32V / 45mA / 19 watts
Output transformer 19:1
I tested with an 8 ohm speaker.
I replaced all the electrolytics.
I measured all the resistors.
I tested with three different sets of tubes.
What am I missing? Maybe all the tubes are bad? Maybe a resistor goes up or down with the voltage? Maybe a voltage sinks in use? A misconnection somewhere? A bad transformer?
A self drawn schematic:
Dropbox - Eko Herald III Bass power amp schematic.png
Plate Voltages 412V
Screen grids 388V
The third node 363V
Bias -32V / 45mA / 19 watts
Output transformer 19:1
I tested with an 8 ohm speaker.
I replaced all the electrolytics.
I measured all the resistors.
I tested with three different sets of tubes.
What am I missing? Maybe all the tubes are bad? Maybe a resistor goes up or down with the voltage? Maybe a voltage sinks in use? A misconnection somewhere? A bad transformer?
A self drawn schematic:
Dropbox - Eko Herald III Bass power amp schematic.png
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A 19:1 transformer loaded by 8 ohms reflects a plato-to-plate impedance of 2884 ohms, which is too small for a couple of EL34's
Best regards!
Best regards!
Ok thanks! So how much should it be? The Philips tube manual says 3400 ohms for class AB. Is that per tube or per pair? If it’s per tube then I’m pretty far off.
So if the math goes like this:
3400 ohms / (19^2) = 9.4 ohms
Is an 8 ohm cab not the way to go? It's less than 20% off. 16 ohms would be way off.
3400 ohms / (19^2) = 9.4 ohms
Is an 8 ohm cab not the way to go? It's less than 20% off. 16 ohms would be way off.
If I remember right my 60 watt quad el34 amp had a bias of about 18 volts but I could be wrong.
Well, you've been reporting an issue, and I've just been assumpting a possible cause.
Best regards!
Best regards!
Yes of course Kay Pirinha! I really appreciate your help! Thank You!
I’m just trying to make sense of the situation because I’m stuck with this repair. I really don’t know how to proceed.
My previous message was more of a question than a statement. Are my calculations correct?
I’m just trying to make sense of the situation because I’m stuck with this repair. I really don’t know how to proceed.
My previous message was more of a question than a statement. Are my calculations correct?
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Use a 16 ohm load and measure power out, what do you get?
You will have some 5800 ohms plate to plate, not bad considering "original" Marshall JTM45 used 6600 ohms.
Tubes are reasonably flexible in load impedance, so The output is weak and distorted. makes me think of 2 possibilities:
1) those tubes are worn to death. Try a new pair.
2) that combined with This amp sounds fine up until the grids of the output tubes. makes me think of a shorted OT.
Do you have a scope?
Can you scope signal into power tube grids and speaker out?
If possible, post some screen images, even phone pictures will do.
You will have some 5800 ohms plate to plate, not bad considering "original" Marshall JTM45 used 6600 ohms.
Tubes are reasonably flexible in load impedance, so The output is weak and distorted. makes me think of 2 possibilities:
1) those tubes are worn to death. Try a new pair.
2) that combined with This amp sounds fine up until the grids of the output tubes. makes me think of a shorted OT.
Do you have a scope?
Can you scope signal into power tube grids and speaker out?
If possible, post some screen images, even phone pictures will do.
Thanks for the help JMFahey!
I don't have a scope. I might even go and buy used one. It could take some time. I'm doing this to learn about amps and good tools are always good to have.
I think I'll go and measure the OT with the multimeter first. It's better than nothing I guess.
The amp has no HT fuse which makes OT damage a bit more probable.
I don't have a scope. I might even go and buy used one. It could take some time. I'm doing this to learn about amps and good tools are always good to have.
I think I'll go and measure the OT with the multimeter first. It's better than nothing I guess.
The amp has no HT fuse which makes OT damage a bit more probable.
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Thanks Nigel!
This amp is very close to a Marshall plexi. Some Marshall plexi schematics show -30 volts bias. I guess I'm not too far off.
Unfortunately a HT fuse usually won't prevent the OT from damage, as the main reason is a cranked amplifier without speaker load. Under this cndition the finals don't draw too much plate current, so a fuse won't open.
Best regards!
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