Yaqin MC-100B heater elevation

[vidax]

Do you think it would be recommended making heater elevation circuits in a Yaqin mc-100b with the following electric diagram:

 

[vidax]

I asked the question because the 6SN7 cathode followers Uk-h is exceeded, as shown in schematic. Is it dangerous for the tubes span life?

 

[trobbins]

I can't see any concern with adding a decoupled divider - perhaps from the Q1 supply, and choosing something like +90V, so the 12AX7 doesn't then get stressed.

I guess you can't switch around the heaters between channels?

 

[vidax]

The amplifier has two power transformers, one for L channel and one for R channel. Each transformer has one single filaments power, feeding tubes per channel is common. If I raise the filaments potential for all tubes is it a hazard for KT88 power tubes?

 

[trobbins]

Have you looked at the datasheets of each of the valves?

Can you identify the heater-to-cathode voltage for the existing setup, and for when you change the heater elevation from 0V to say +90V?

Is the amp working ok at the moment?

Can you rewire heaters from separate valves to the other heater supply easily?

 

[vidax]

In the datasheet of 6SN7 the max. Uk-h is 200V.
I was worried about voltage values of the schematic that I found on the web.
Single measurement was made with final tubes, KT88, removed from sockets, I measured in a socket, for 232Vac network I found plate voltage 465Vdc and heater voltage 6,9Vac. The amplifier works well, I have it about three years. Over time I rolled-on all the tubes. I had problems with some 6SN7 JJ, after warming they crackling and humming. In the same places 6SN7 Tungsol work well. After I'm finding the best solution I'll rewire heaters, I can do this.

 

[petertub]

In the datasheet of 6SN7 the max. Uk-h is 200V.
I was worried about voltage values of the schematic that I found on the web.
Single measurement was made with final tubes, KT88, removed from sockets, I measured in a socket, for 232Vac network I found plate voltage 465Vdc and heater voltage 6,9Vac. The amplifier works well, I have it about three years. Over time I rolled-on all the tubes. I had problems with some 6SN7 JJ, after warming they crackling and humming. In the same places 6SN7 Tungsol work well. After I'm finding the best solution I'll rewire heaters, I can do this.

Yes many datasheets say 200V, but a few :
https://frank.pocnet.net/sheets/191/6/6SN7GTB.pdf ( RCA )
says that the DC/average should not exceed 100V.
Cracking and/or humming is a symptom of broken down isolation between
filament and catode.
Go for elevated filemanets, get a separate filament transformer if needed.

 

[trobbins]

vidax, what did you identify from for the other two tube datasheets?

Can you calculate what the voltage difference is between cathode and heater for each of your tubes - if you use the voltages on that schematic?

 

[vidax]

In electronic diagram the power outlet median filament winding is grounded. Filaments potential is almost 0V (actually 3,15V) so voltage between cathodes and filaments are approximately those given in scheme, 283V for cathode followers, 173V for phase inverter, for second SRPP cathode can be calculated taking into account the internal resistance of the tube. To find exact values I will remove cover and will do measurements. I think the solution is to realize two separate filament transformers. Thanks.

 

[trobbins]

Perhaps another heater transformer (6V 2.4A or 12V 1.2A) for the 6SN7's.

It would also be worth elevating the existing heaters, to reduce stress on the 12AX7's, as the KT88's won't mind.

 

[vidax]

I have decided to keep the original heater supply for KT88 valves since the voltage between filament cathode is 500mV and to make heater elevation for 6SN7 and 12AX7 valves. For this I made two toroidal transformers, one for each channel. For each, primary winding fed to 6,3V to keep consumption and for easier winding and secondary : 2x3,15V, 2x6,3V and 2x7,5V. In this way for the filament supply of 12AX7 tube I will be able to use 6,3V ac or 12,6V ac, the median filament grounded, or 12,6Vcc stabilized.
These are transformers:

 

[DavidT]

In the schematic, why is C6 connected to the -70V bias, r13, and consequently the 6sn7 cathode of Q3...?

 

[vidax]

Scheme is drawn wrong, so I found it on the web. Voltage values are incorrect also. I'll give the schematic with the correct values measured by me after removal device:

 

[vidax]

This is the change in the amplifier schematic:

 

[vidax]

Transformers test before the final mounting:

 

[DavidT]

Thank-you Vidax; your circuit correction matches my understanding. (As you mentioned in your profile, I also designed automation equipment - great fun!)

 

[vidax]

Because voltage network at my house varies between 232...240 V, filament voltage reached 6,9...7V, I realized transformers with a transformation ratio slighty below par and in KT88's filaments supply lines I tied resistors in series. Now filament voltages not exceedind 6,5V. Other modifications: have rotated 90 degrees power transformers, have mounted EMI filter to the mains supply, have mounted a soft-start circuit. All changes can be seen in photo:

 

[vidax]

David, have you too an Yaqin amplifier?
A picture that shows how I turned power transformers:

 

[DavidT]

No Yaqin amplifier here, but if I did have one, I would probably do the type of changes you are implementing on your amp...nice work!

My current project, which has been interrupted by our bathroom remodel (what!), is a pair of new monoblocks: 6CG7 voltage amp - 12BH7 LT Phase splitter - KT88 output. Transcendar and Hammond transformers.

New construction sure is time-consuming, but also rewarding... Generally, I prefer to modify existing designs, like you are doing here.

 

[vidax]

And I intend to replace the Chinese output transformers with Hammond 1650NA transformers. I guess it sounds better.