Help with the modifications of my tube-amp

[murphythecat8]

hi everyone.
I have bought a diy tube amp and I need to modify a couple of things.

the schematic is attached.

Thorsten loesch have given me a lot of help and mostly told me what to do. I need however guidance.

I need to change:

1) The output tubes are 6.3V heater types, directly heated.

There are a number of effects in directly heated tubes, that mean AC heating derived problem increase by the square of the Voltage.

So while AC heating the 2A3 at 2.5V can work okay especially in a PP Amp, the problems are >20dB greater with 6B4 type valves.

The sonic problems from AC include a warm, but muddy and not very detailed sound, often lacking subjective dynamics.

One solution would be to change the heaters to DC (not straightforward, bad DC on directly heater tubes sounds even worse than bad AC, but in different ways).



2) The output transformers as said, are unsuited for operation without feedback by design.

It would probably be a good idea to add some, around 10dB should help to open up the bandwidth.

Can someone guide me with those mods?

thanks

[Osvaldo de Banfield]

Hi boy!!!
Look at this: AC heaters had been used for decades, so I don´t believe to be necessary to get DC on them. And the trouble is augmented if the same transformer is being used, because you must drop out the difference between the AC RMS voltage and the DC voltage (1.414 times greater).

And second, the amplifier mus be designed to support NFB. The output transformer mainly, can cause a phase rotation that may cause the amplifier to convert into an oscillator.If the amplifier entirely has not be designed to NFB used, the phase rotations had not being into account, and may be impossible to use with NFB. Read some data in the web relative to NFB, and you surely will agree with me.

Best regards and good luck!

[Palustris]

Originally Posted by murphythecat8

hi everyone.
The sonic problems from AC include a warm, but muddy and not very detailed sound, often lacking subjective dynamics

Is this something you are hearing or is this theoretical? I have several PP 6B4 amps and don't experience these problems.

You do have a problem with your filaments, however. You have both the output tubes and the front end tubes all connected to the same filament transformer. As such your 6V heater supply is being modulated by the output tubes through their cathode connection. You need to add a filament transformer for either the OP tubes or the front end tubes so that they are separate.

Rod Coleman who posts on this board has voltage regulator kits for DHT filaments if you think that's what you need, but at least decouple the front end tubes from the OP tubes first.

[tomchr]

Originally Posted by Palustris

You do have a problem with your filaments, however. You have both the output tubes and the front end tubes all connected to the same filament transformer. As such your 6V heater supply is being modulated by the output tubes through their cathode connection. You need to add a filament transformer for either the OP tubes or the front end tubes so that they are separate.

The input tubes are indirectly heated, hence, there's no problem with sharing the heaters for the input tubes with the filaments on the output tubes. The cathodes of the output tubes are bypassed by C6, hence, there will ideally be no AC component in the audio range. In reality, there will be a small AC component but it should be small enough to not interfere with the input tubes.

You can use a DC supply for the heater, but you'll find that all solutions require a higher DC input voltage than what you can get with a 6.3 V winding. Some regulators burn more power in the regulator than they deliver to the tube filaments -- something I've always found a bit silly. Hence, when I designed my DC filament regulator, I opted to use a switchmode supply. I have boards available on my website (see my signature).

~Tom

[DavesNotHere]

change the output tubes to 6a5G as this is a 2a3 indirectly heated.
and
remove r13, r14 . change r11 and r12 to 100 ohms 1W change c6 to .1 uf 250V poly

The best results with DHT I found is using a separate heater supply that is center tapped, and put the cathode circuit on the center tap. with your setup I would reuse the 1K resistors tied in parallel and bypass with the 220Uf cap and put a .22 uf poly or pio cap and maybe even another 220 uf cap just to lower the impedence of the AC ground (pathway through the capacitors). The trick is to find a good transformer that has been built nicely as the center tap has to be perfect. It easy to get there with a toroid type transformer. its about 1 out of 3 good hammonds if you want to hand pick a more traditional style transformer.

[speaker]

Originally Posted by DavesNotHere

change the output tubes to 6a5G as this is a 2a3 indirectly heated.

Do you have a source for 6A5's of any type?

[speaker]

Originally Posted by murphythecat8

2) The output transformers as said, are unsuited for operation without feedback by design.

It would probably be a good idea to add some, around 10dB should help to open up the bandwidth.

The A470 output transformer is good iron. I think the comment on them needing feedback is more a function of their original use on the ST70 which used pentodes for the finals. IIRC, you can replace the EL34's on an ST70 directly with 6B4G's and disconect the feedback entirely with good results. (Someone will comment if I am all wet on that.)

[Palustris]

Originally Posted by tomchr

The input tubes are indirectly heated, hence, there's no problem with sharing the heaters for the input tubes with the filaments on the output tubes. The cathodes of the output tubes are bypassed by C6, hence, there will ideally be no AC component in the audio range. In reality, there will be a small AC component but it should be small enough to not interfere with the input tubes.

This is all technically true, however, I just find that it is preferable to have the OP tubes on a separate filament transformer.

Originally Posted by DavesNotHere

change the output tubes to 6a5G as this is a 2a3 indirectly heated.

The 6A5G is really a 6B4G with the center of the filament connected to a cathode. This was done to reduce hum.

Originally Posted by DavesNotHere

The best results with DHT I found is using a separate heater supply that is center tapped, and put the cathode circuit on the center tap. with your setup I would reuse the 1K resistors tied in parallel and bypass with the 220Uf cap and put a .22 uf poly or pio cap and maybe even another 220 uf cap just to lower the impedence of the AC ground (pathway through the capacitors). The trick is to find a good transformer that has been built nicely as the center tap has to be perfect. It easy to get there with a toroid type transformer. its about 1 out of 3 good hammonds if you want to hand pick a more traditional style transformer.

I agree.

[murphythecat8]

Originally Posted by Palustris

Is this something you are hearing or is this theoretical? I have several PP 6B4 amps and don't experience these problems.

well its something that was suggested to me. I dont like the sound of the amp so Im looking to what to do. I do need to change some key caps to better ones but I want to focus on the design flaws first....

Originally Posted by DavesNotHere

The best results with DHT I found is using a separate heater supply that is center tapped, and put the cathode circuit on the center tap. with your setup I would reuse the 1K resistors tied in parallel and bypass with the 220Uf cap and put a .22 uf poly or pio cap and maybe even another 220 uf cap just to lower the impedence of the AC ground (pathway through the capacitors). The trick is to find a good transformer that has been built nicely as the center tap has to be perfect. It easy to get there with a toroid type transformer. its about 1 out of 3 good hammonds if you want to hand pick a more traditional style transformer.

any suggestion of which separate heater supply? I would however want to keep the 6B4 to keep the mods cost as low as possible...

Originally Posted by Osvaldo de Banfield

And second, the amplifier mus be designed to support NFB. The output transformer mainly, can cause a phase rotation that may cause the amplifier to convert into an oscillator.If the amplifier entirely has not be designed to NFB used, the phase rotations had not being into account, and may be impossible to use with NFB. Read some data in the web relative to NFB, and you surely will agree with me.

I guess I shouldnt bother to add feedback then? If I apply NFB correctly shouldnt I gain something?

[tomchr]

I'd use a 2x12 V 25 VA transformer from Antek Inc. with my regulator board... But I'm biased, obviously.

You could get a 2x6 V, 25 VA transformer and add a few windings to get to 6.3 V if you want to stick with AC heating.

~Tom