[colinharding]

Perfect thank you! And henceforth it shall be called amplification stage in my references

 

[adeadcrab]

The only change I would want is to extend the very top end (>10khz) somehow, which I have to imagine is a limitation of the tubes themselves (Russian 6H13C + 6SJ7GT mesh, a very clean and resolving pairing). Not sure is changing the load resistors would yield that result.

 

[simmconn]

The specimen I tested has flat response (-0.1dB) from 22Hz to about 46KHz ref. 1kHz, at 8V into 300 Ohm (almost full power). I don't see that the top end is restricted. Different tube combinations may have different distortion profiles which affect the perceived FR flatness, though. A lower load resistor will result in better top end response at a cost of slightly increased distortion, but given the decent performance of the original circuit, the improvement might not be discernible.

 

[adeadcrab]

The specimen I tested has flat response (-0.1dB) from 22Hz to about 46KHz ref. 1kHz, at 8V into 300 Ohm (almost full power). I don't see that the top end is restricted. Different tube combinations may have different distortion profiles which affect the perceived FR flatness, though. A lower load resistor will result in better top end response at a cost of slightly increased distortion, but given the decent performance of the original circuit, the improvement might not be discernible.

I tested last night, compared to the output of my Soekris 1421 the very top end was not as sparkly, not much of a difference but something I picked up on. May just be the 6H13C tube though.. the GE 6AS7GA is sounding almost identical to the DAC output!

I'll be printing out your thorough post earlier and handing it to my tube amp guy sometime this year, will mention this lower load resistor as a potential mod too. Thanks for helping a longtime 339i user with some practical upgrade tips!

 

[hp4fun]

How to stop tube hum? It started quiet and then gradually became louder. I can stop it by tapping the tubes but won't last long. Tried the tube rubber band but did not help.

 

[colinharding]

Which set of tubes is it? I’ve certainly had some tubes develop microphone overtime.

 

[hp4fun]

Vt-116. Also not sure why tapping stops the hum

 

[colinharding]

Yeah this is fairly common in my experience unfortunately. There are quite a few elements within any tube that can move around over time, if only ever so slightly.

Humming likely indicates that something has moved slightly from its original position and why you're tapping on it seems to help as it jostles it back to where it was. Nothing out of the ordinary, would just look for another tube or tap on it when you need to so the hum goes away

I've had this with 12AX7, 6DJ8, 300B, etc. Could also be a symptom of a tube nearing the end of its life though I assume you bought these NOS so unlikely. I do have a tube tester if you want to find out

 

[hp4fun]

Vt-116. Also not sure why tapping stops the hum

I sort of figured out it is due to the loose base. Tried to reflow soldering the pins but doesn't seem to work well (my soldering tool is for kids anyways).

 

[colinharding]

I guess the base could cause the issue. Not seen that one before unless it's also somehow moving some of the internals when it moves.

Think I'd just get another tube if I were you. The JAN 6SJ7 are quite cheap. The 'red hot' version of this also sounds quite a bit better, may be worth just moving in that direction.

 

[hp4fun]

I guess the base could cause the issue. Not seen that one before unless it's also somehow moving some of the internals when it moves.

Think I'd just get another tube if I were you. The JAN 6SJ7 are quite cheap. The 'red hot' version of this also sounds quite a bit better, may be worth just moving in that direction.

VT 116a is 6sj7gt, designated code during WW2 iirc. It sounds better than red hot with GEC series.

 

[colinharding]

Ha gotcha - had thought these were the metal encased versions of the tube.

Makes a bit more sense as to the humming and loose base now. Apologies for my initial confusion. Have you tried these tubes against the mesh Tung Sol version by chance?

 

[hp4fun]

Ha gotcha - had thought these were the metal encased versions of the tube.

Makes a bit more sense as to the humming and loose base now. Apologies for my initial confusion. Have you tried these tubes against the mesh Tung Sol version by chance?

Both works quite well with the GEC CV 4079, in a different way.

Mesh is a bit relax and mellow. Solid is more intense. But it could just be the difference between these two sets.

 

[colinharding]

Thought I'd add some new pics - changed the volume knobs

That's about all I can do while the soldering station and the rest of my tools are packed waiting to move into the new house. Can't wait to get started on installing the rest of the components I've received for this amp.

 

[hp4fun]

For those who know my previous posts, I tend to do very cheap and low tech black box and possibly random experiments. Some were stupid and some were fun. This might be both.

I got rid of the hum of my 6sj7gt by..... lifting it up to the sky!

I honestly don't know what was changed and it is your time to laugh now.