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Ok, I have looked around online trying to get the answer to this question. Are TC-15 and the variants class A or AB. There doesn't seem to be a definitive answer to this, at leastI can't seem to find it. Anyone ?

Class AB, no argument.

Just because an amp is biased hot (and these amps are) does not mean they are Class A.

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http://pdfelectronics.com

Thanks Kurt! I thought as much.A lot of builders seem to throw catchphrases and buzz words around and don't really know what is what,I think.The interwebs is chock full of misinformation. I converted a Hammond AO -43 to a Trinity 15 style amp and was trying to get the voltages right by using the BBQ/Jim Jones bias guide. I haven't ever worked on a AC 15/30 /Matchless type circuit,so I wanted to get my info straight so as not to blow things up or let the smoke out!

Not my work but here you go ... This holds true for the AC15 , Matchless , Top Hat! Bad Cat etc. etc..


The Vox AC30 has always been considered a 'pure' Class A design, and is the first example offered when someone inquires about what a Class A amplifier 'sounds like'. Unfortunately it is not a complete Class A amplifier, and the confusion stems from the fact that it is using cathode biasing on the output tubes. By definition, any Class A amplifier draws' maximum' plate current at all times, and it should not deviate between idle (quiescent) and full output power. Another way of saying this is that the 'maximum' plate current is flowing at all times. The Class AB amplifier will have its plate current increase approximately 25% between idle and full output, while each tube takes turns 'not conducting'. The same plate current is said to be flowing between 'appreciably more than half and less then the entire electrical cycle'.1 Studying a schematic and taking voltage and current measurements when you have the opportunity will help you understand things a little better. Lastly, by looking at a tube manual for the 6BQ5, we see that a Class AB amplifier needs a common 130-ohm resistor to achieve an output of 17 watts (4% THD), with the other circuit parameters taken into account, including a quiescent plate current of 72mA and maximum plate current of 92mA (both tubes). Do the math and check out that the AC30 has a common 50-ohm resistor biasing four EL84's. Even the schematic states that the voltage measurement at the cathodes should be 10VDC at quiescent and 12.5VDC at full output. That works out to approximately 200mA at idle through four EL84's and 250mA at full output. By their own (not very) silent admission, the AC30 is not even close to a true Class A amplifier.] Vox AC30 has always been considered a 'pure' Class A design, and is the first example offered when someone inquires about what a Class A amplifier 'sounds like'. Unfortunately it is not a complete Class A amplifier, and the confusion stems from the fact that it is using cathode biasing on the output tubes. By definition, any Class A amplifier draws' maximum' plate current at all times, and it should not deviate between idle (quiescent) and full output power. Another way of saying this is that the 'maximum' plate current is flowing at all times. The Class AB amplifier will have its plate current increase approximately 25% between idle and full output, while each tube takes turns 'not conducting'. The same plate current is said to be flowing between 'appreciably more than half and less then the entire electrical cycle'.1 Studying a schematic and taking voltage and current measurements when you have the opportunity will help you understand things a little better. Lastly, by looking at a tube manual for the 6BQ5, we see that a Class AB amplifier needs a common 130-ohm resistor to achieve an output of 17 watts (4% THD), with the other circuit parameters taken into account, including a quiescent plate current of 72mA and maximum plate current of 92mA (both tubes). Do the math and check out that the AC30 has a common 50-ohm resistor biasing four EL84's. Even the schematic states that the voltage measurement at the cathodes should be 10VDC at quiescent and 12.5VDC at full output. That works out to approximately 200mA at idle through four EL84's and 250mA at full output. By their own (not very) silent admission, the AC30 is not even close to a true Class A amplifier.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks Stephen.I recall reading that info somewhere a while back... Aiken site maybe?
Anyways I pretty much finished this AO-43 conversion to something similar to the Trinity 15 variant(with a master vol bypass and cut control. By the use of Zeners and a 5u4,I managed to get the B+ voltage tamed down to around 346 volts.All the voltages for the 12ax7s are close to your specs. My plate voltages for the el84s(new Mullards) are 334 and 337 respectively and 337V at the node where the 2 big 100R's meet, but when I check the voltage at the 120R /220uf cathode bias node(pins 3 on the el84s,I am getting 12.33 volts and 12.38 across the 120r . It seems a bit high to me.Is it ok or too high? This whole bias thing has my head spinning!

Oh yeah, I also reused the choke that came with the Hammond- any idea what the specs are for that in Henries etc?Impossible to find that info online!

With a cathode voltage of 12.3V and a plate voltage of ~330V you're dissipating 16W of power on the plates. Your running the amp at 133% plate dissipation (49mA plate current per tube). Sounds high but it's not terrible.

Due to the NFB nature of cathode biased output stages that plate dissipation number drops and evens out when the amp is being played and not at idle. Though the NFB nature of the output stage is slightly limited due to the high bypass cap (but I wouldn't change it as it helps tone).

Anyways I'd increase the cathode resistor a bit. Try 150R 5W and remeasure your voltages.


The specs of the choke don't matter at all really. 2H vs 10H is meaningless in tube amps. All you need make sure of is that your choke can handle the screen (and downstream) current. Sounds like your good though!

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http://pdfelectronics.com

Thanks Kurt. I will try that and see. I noticed that some other similar circuits may use up to 1K screen resistors.What would change by subbing different values there?

Ok, I did some changes. I added one more 22uf 450V cap that supplies the screen node. I was using the Ceri@tone setup for caps(don't ask me why! ) That circuit uses one less filter cap than the Trinity 15,so now there are 2-33ufs and 4-22ufs . I changed the cathode resistor to 150R and now I get 8.27V at the cathode point,341 and343 volts on pins 7 of the el84s and 212V at the 100R junction node.Does this give me 8watts of power on the plates and 25mA plate current per tube? I am using the BBQ bias calculator to figure this out,it doesn't seem really clear to me I guess.So many variables!

Yes roughly 8W per tube, correct.

I am curious why your screen voltage is so low. What is the path leading up to the screen resistor?


Larger values for the screen resistor drop more voltage when the amp is being played hard and screen current increases. A larger value resistor will cause a softer sound with a bit more compression I have found. Worth experimenting if you're curious...

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http://pdfelectronics.com

Great! I am getting somewhere! The screens are wired liked the TC15. the first 22uf on the left supplies the two 100Rs. Stephen shows 184 V at that point-it seemed right to me.If I bypass that 22k resistor,I get 12.35 v at the cathode and the screen voltage goes up to about 317V and the plates go down to 318V.