Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JohanH on January 19, 2020, 11:51:27 am
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Hello! Im currently building a 6g2 with an old PT. The transformer is tapped as follows
Pri:
130V
220V
Sec:
435-0-435 HV
6,3 V heater filament
5V rectifier filament
Currently, the amp measures about 580V on the B+, way high for a 6G2.
What Id like to do is effectively halve the HV voltage.
Looking at Rob Robinettes site, I found the following schematic for a tube rectifier without a PT center tap.
(https://robrobinette.com/images/Guitar/5E3P_Build/Hybrid-Rectifier.gif)
Could this be used to create a "new" HV on my PT?
Connecting one HV lead and the CT to the rectifier, and isolating the other HV winding. Essentially, this would give me a 217-0-217 winding.
Once rectified, it would probably give me about 320-350 Vdc on B+.
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Using that FWB with only half the secondary winding will produce the same B+ as using the whole center tapped winding with a two diode conventional rectifier. You really need a more appropriate transformer.
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Using that FWB with only half the secondary winding will produce the same B+ as using the whole center tapped winding with a two diode conventional rectifier. You really need a more appropriate transformer.
Theres a 5Y3 in the amp, its already full wave recitified
Is disconnecting one plate of the 5Y3 an option?
I suppose I could modify the circuit to run on higher B+. The measured 580V is without tubes, and modern 6V6 JJs run at 500V. With a reasonably sized zener and a choke, the B+ could be lowered even more. Just have to find a way to reduce the voltage to 300v for the preamp.
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Is disconnecting one plate of the 5Y3 an option?
NO!
Just build the amp with that PT and see where the B+ ends up once the tubes are in and it's running properly. Might not be as bad as you think.
Dropping down to 300V for the preamp is easy, just use a larger dropping resistor for the preamp node.
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Is disconnecting one plate of the 5Y3 an option?
NO!
Just build the amp with that PT and see where the B+ ends up once the tubes are in and it's running properly. Might not be as bad as you think.
Dropping down to 300V for the preamp is easy, just use a larger dropping resistor for the preamp node.
Alright, I'll throw a pair of JJs in it tomorrow to see where it lands.
How much should I expect to see the B+ drop?
Doing some reading, not many 6G2 amps seem to have the 315 B+ that is written in the original fender schematic.
Im still learning, and on a students income Id like to avoid buying a new PT!
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How much should I expect to see the B+ drop?
No way to tell. It depends on several things, PT current rating, how hot you will bias the tubes, what type 5Y3 you have, etc.
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How much should I expect to see the B+ drop?
No way to tell. It depends on several things, PT current rating, how hot you will bias the tubes, what type 5Y3 you have, etc.
Thanks for the help, its appreciated :occasion14:
A few pictures of the amp as of now
(https://i.imgur.com/67Ka9aB.jpg)
(https://i.imgur.com/zDjGDTp.jpg)
Oh and right! Im using a pair of 6SL7 in the preamp instead of 12ax7.
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What were the original power tubes?
Did you remove a choke?
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What were the original power tubes?
Did you remove a choke?
Original power tube was a single 6L6GT.
I removed a small choke right after the rectifier. When attached again, I see the a voltage drop of about 10v in the unloaded B+ values
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> a small choke right after the rectifier. When attached again, I see the a voltage drop of about 10v in the unloaded B+ values
I *think* this must have been a Choke-Input filter. Factor 0.9 instead of 1.4. The choke action does not happen at no-load, you need to load it.
Taking 435VAC, times 0.9, makes 390V. Deduct 50V for rectifier, you are at 340V, a reasonable place to work one 6L6. But NO-load it will still soar to a frightening 600V.
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> a small choke right after the rectifier. When attached again, I see the a voltage drop of about 10v in the unloaded B+ values
I *think* this must have been a Choke-Input filter. Factor 0.9 instead of 1.4. The choke action does not happen at no-load, you need to load it.
Taking 435VAC, times 0.9, makes 390V. Deduct 50V for rectifier, you are at 340V, a reasonable place to work one 6L6. But NO-load it will still soar to a frightening 600V.
I'm a bit stumped, with power tubes and choke the voltage is steady at 580 V. Does the bias voltage play into this equation? I measured it as of now and it's -55 V.
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For the choke input circuit to work to reduce voltage, the choke must be connected directly to the rectifier. There must not be a filter cap connected directly to the rectifier. And the OT plate lead will be connected AFTER the choke, it cannot be connected to the rectifier.
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Sorry - A slight wiring error (forgot to ground the cathodes) was the reason that the b+ stayed high with tubes. It's now at 250, without filter choke. Still waiting for a pair of 6sl7 so I can test voltages with appropriate load.
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The power tubes are the main load. The 6SL6s will not affect the B+ very much.
So, you've gone from 580V unloaded to 250V with power tubes installed and no choke? That must be a very wimpy PT.
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The power tubes are the main load. The 6SL6s will not affect the B+ very much.
So, you've gone from 580V unloaded to 250V with power tubes installed and no choke? That must be a very wimpy PT.
Spec wise atleast - it's pretty large for this size amp. I'm figuring a diffrent rectifier might help it.
I took some voltages and tried to set a bias for the amp, but I came up with a 14V drop across the plate to CT on each output tube :think1: Divided by the primary resistance, that gave me a plate current of about 90 mA (???)
The tubes where not red plating, and 90mA is way beyond a 6v6... something must be very wrong :w2:
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The pic of your amp seems to show some wiring errors and some missing wires too. For example, the wire from the volume pot looks to be connected to the wrong side of the coupling cap. And cap is missing between the volume and tone control. I can't see any connection between the bias board and the 220K grid resistors on the main board. There's a 100K resistor across the bias cap that should be a 30K. Lot of stuff that's different from the 6G2 schematic.
What voltage do you have on pin 5 of each 6V6 socket?
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There is a way to add a bleeder resistor, uncle doug reviewed a super old 1930s/40s amp that used a high wattage resistor to bleed exceas voltage to ground, i think it was at the node of the p.s .but i will try to see how it was done and report back. It effectively brought down the voltage quite a bit
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There is a way to add a bleeder resistor,
Yes - this can work. And I think Sluckey has done so on some of his builds. I have done it a couple of times, once on a B+ node and once on the center tap. But depending on the value, the resister needs to be really high wattage - I used 30W. And you need to be careful about the heat generated - I have the chassis acting as a heat sink.
Last - I built a 6G2 and believe the lower B+ voltage to be one of the things that sets it apart from the blackface Princeton, so I would join on suggesting saving that PT for another project and getting PT designed for the 6G2 requirements. I used one of the ClassicTone PTs.
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The pic of your amp seems to show some wiring errors and some missing wires too. For example, the wire from the volume pot looks to be connected to the wrong side of the coupling cap. And cap is missing between the volume and tone control. I can't see any connection between the bias board and the 220K grid resistors on the main board. There's a 100K resistor across the bias cap that should be a 30K. Lot of stuff that's different from the 6G2 schematic.
What voltage do you have on pin 5 of each 6V6 socket?
I hadn't double checked the wiring, Its been corrected now.
I get about 220V for the screen circuit, pin 5 of each 6v6.
A bit low, no?
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I get about 220V for the screen circuit, pin 5 of each 6v6.
A bit low, no?
No wonder the tubes are on fire! Pin 5 should have about -35V.
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I get about 220V for the screen circuit, pin 5 of each 6v6.
A bit low, no?
No wonder the tubes are on fire! Pin 5 should have about -35V.
Sorry, pin 4 is 220. Pin 5 is floating, and not connected to the bias supply
I'll correct it today and expect to see the B+ rise, when the tubes are not drawing 90mA each :occasion14:
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Biased to about 18mA draw at 450 plate Voltage, seems to work ok now!
I tried the choke input to bring the plate voltage down.
With correct bias and no choke the plate voltage landed at 490v, now a nice 450v.
A larger dropping resistor should bring preamp b+ to 300v, no? If not, I might throw a pair of 6L6GT in there to draw a bit more current.
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A larger dropping resistor should bring preamp b+ to 300v, no?
yes
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(https://i.imgur.com/k1LkEKg.jpg)
Amp is working (and sounding) great now! Plate voltage is about 450 at the 6V6s, 280 at the 6SL7s.
The amp has a lot of gain and responds nicely to volume-control interaction. Cathode bypass cap for v1 is switchable and the negative feedback is adjusted via a pot at the back
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:thumbsup:
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The amp has a lot of gain and responds nicely to volume-control interaction.
You used 6SL7 tubes instead of 12AX7 (7025) tubes in the pre
I would like to know
did you maintained the same components values around the preamp tubes ?
Thanks
Franco
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The amp has a lot of gain and responds nicely to volume-control interaction.
You used 6SL7 tubes instead of 12AX7 (7025) tubes in the pre
I would like to know
did you maintained the same components values around the preamp tubes ?
Thanks
Franco
No, I used a 2,2uf (instead of 22uf) cathode bypass cap and the standard 1,5k cathode bias resistor for V1.
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:thumbsup:
Big thanks for all the help! Much appreciated :occasion14:
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Thanks
Franco