Hello folks,
I have this amp that uses 2ea 6V6 tubes normally connected as Push-Pull config cathode biased. The amp is designed with a switch that disconnects one of the cathodes for approx 1/2 power and then just uses 1/2 of the output xformer. The coupling caps to the grids are not leaking. The amp is relatively new.
In the 1-tube operation mode, the one tube is drawing 92ma current and with plate voltage of 440Vdc, that makes 40watts at idle!! I raised the cathode resistor from 250ohms to 350ohms to really not change anything substantially. There is no bias voltage circuit to augment the cathode bias...Yes, I've seen circuits that have both! This is a self-bias type of configuration, so you can only do so much xnging the cathode resistor value (BTW, the bypass cap is good).
I'm thinking that the plate & screen voltages are just too high for a single ended 6V6, but I'm not a designer so I'm a bit at a loss. Screen voltages are about what the plate voltages are.
Is this just a crappy design trying to make a single ended amp out of 1/2 of the output transformer with too high plate voltage? From the RCA tube book, the max plate voltage for a single-ended 6V6 is around 315Vdc. Yes, it is designed for 6V6 tubes. I don't have a schemo, but the output stage is fairly standard.
Any suggestions would be appreciated.
Evil Robot Tone Americana Red Plating
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Mars_Amp_Repair
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Stevem
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Re: Evil Robot Tone Americana Red Plating
That’s criminal that such an amp has been produced and sold!
It’s really the level of screen voltage that controls the current thru the tube.
I would if you could wire up a way with another switch to possibly power the screen off of a low voltage later supply node and increase the wattage of the resistors up to that node to handle that current draw.
Just so you know Blackface and silverface champs and Vibrochamps like mine run over 400 volts on the plate .
It’s really the level of screen voltage that controls the current thru the tube.
I would if you could wire up a way with another switch to possibly power the screen off of a low voltage later supply node and increase the wattage of the resistors up to that node to handle that current draw.
Just so you know Blackface and silverface champs and Vibrochamps like mine run over 400 volts on the plate .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Mars_Amp_Repair
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Re: Evil Robot Tone Americana Red Plating
Ah, I see.
I see where someone suggested adding 1K resistors in series with the screens. Kinda standard for most 6L6 circuits.
I always thought that was for isolation to prevent possible oscillations. Regardless, I'll have to give that a try.
Thanx, Glen
I see where someone suggested adding 1K resistors in series with the screens. Kinda standard for most 6L6 circuits.
I always thought that was for isolation to prevent possible oscillations. Regardless, I'll have to give that a try.
Thanx, Glen
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Stevem
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Re: Evil Robot Tone Americana Red Plating
Wait a minute here.
If the amp in normal mode is A/B then how is that when the amp is set to run single ended is that one tubes input grid then is receiving the full signwave?
That's a basis of class A!
What changes with the PI section when that's set for single tube supposid class A?
If the amp in normal mode is A/B then how is that when the amp is set to run single ended is that one tubes input grid then is receiving the full signwave?
That's a basis of class A!
What changes with the PI section when that's set for single tube supposid class A?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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statorvane
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Re: Evil Robot Tone Americana Red Plating
P1eXtreme ran a 6V6 at around 370 Volts on the plates with a 600 Ohm cathode resistor. Might try raising the cathode resistor to 600 Ohms.
Re: Evil Robot Tone Americana Red Plating
The grid of the output tube always receives a full sinewave. Doesn't matter if the output is single ended or pushpull.
Re: Evil Robot Tone Americana Red Plating
How do they expect any worthwhile signal in single tube mode, considering too high a voltage and a small transformer without a gap that's saturated with 92mA?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Evil Robot Tone Americana Red Plating
Gutshot photo of the amp with enough detail to see the circuit?
As I understand it, these are supposed to be clones of the Magnatone 214 from the 1950's.
If there's a PP pair of 6V6 with a 250-ohm cathode resistor, one would really need about twice that to operate in single-ended mode. Generally one would implement with a cathode resistor and bypass cap for each output tube if the idea was to offer single-ended operation.
Would like to see some photos of the amp inside and out if you can manage it.
As I understand it, these are supposed to be clones of the Magnatone 214 from the 1950's.
If there's a PP pair of 6V6 with a 250-ohm cathode resistor, one would really need about twice that to operate in single-ended mode. Generally one would implement with a cathode resistor and bypass cap for each output tube if the idea was to offer single-ended operation.
Would like to see some photos of the amp inside and out if you can manage it.
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Mars_Amp_Repair
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Re: Evil Robot Tone Americana Red Plating
Hey folks,
You're basically all correct to some extent.
I dicked around with this thing and determined the only way it was going to work was to install a DPDT toggle switch that would switch both tubes in with the 250 ohm cathode resistor and in the class A position cut the two cathodes apart and connect a 500 ohm resistor in the now single- ended tube.
Turns per the owner, he sent the amp back to the company that built it to have what what they termed a special 1watt 'Hack' that would do the trick.
I guess they didn't bother to notice that it caused the one 6V6 when in class A to red plate.
The owner ended up just replacing the one tube fairly often. They had no solution for him.
How F-ing stupid and irresponsible.
He finally decided just to have new defeat the 'Hack'.
Man!
Thanx all for the insight and info!
Glen
You're basically all correct to some extent.
I dicked around with this thing and determined the only way it was going to work was to install a DPDT toggle switch that would switch both tubes in with the 250 ohm cathode resistor and in the class A position cut the two cathodes apart and connect a 500 ohm resistor in the now single- ended tube.
Turns per the owner, he sent the amp back to the company that built it to have what what they termed a special 1watt 'Hack' that would do the trick.
I guess they didn't bother to notice that it caused the one 6V6 when in class A to red plate.
The owner ended up just replacing the one tube fairly often. They had no solution for him.
How F-ing stupid and irresponsible.
He finally decided just to have new defeat the 'Hack'.
Man!
Thanx all for the insight and info!
Glen
Re: Evil Robot Tone Americana Red Plating
Amazing.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Evil Robot Tone Americana Red Plating
One suspects the builder just copied the schematic and layout of the Magnatone/Troubador amp without understanding it.
The reason is PhilX used the real deal and that made a market for the clones.
The reason is PhilX used the real deal and that made a market for the clones.
Re: Evil Robot Tone Americana Red Plating
I've heard of better way to turn a push pull amp into a quasi single ended is with a pot or mute switch that turns off the control grid signal to one of the power tubes. That leaves the idle current flowing through both halves of the OT (and both tubes' current through the single cathode resistor in this case).