This is the sound of one of the tubes Red-plating in a Chinese Amp. The tubes were all Chinese so I replaced the output tubes with Russian matched pairs and got a better sound from the mid rage with a little less Bass a response that I was not happy with. So I ordered 8 of the Russian equivalent tubes (6P1P) and swapped out the matched pairs and the sound was better. Still lacking low end Bass though , So I thought I would change the Pre-amp tubes for General Electric 6U8A'S and what a difference that made. Overall the amp now sounds much better, more uniformed across low mid and top frequencies with real punchy Bass. The Chinese tubes lasted about 18 months which is not bad as I use this amp in my workshop at 3/4 volume (4/5 with Jimmy Hendrix playing Live) while redoing my test setup and building other amps. Chinese Amp came all built up but then I went and Bought the Boyuurange A9 in kit from as it was so cheap. Thought I would see the difference between Class A/B and Class A output stages. I completed the build and tested.
This amp sounded terrible . The mains Hum was very loud with no Bass . Dam Chinese rubbish .
I preceded to find out were the Hum was coming from and found out that were the tubes are mounted there was no Ground at the metal plate that the tubes are all mounted on although the plate is secured with bolts to the main chassis . I grounded the plate and this made no difference. I then added 100ohm virtual grounding resistors to the heater supply to the output tubes , still no difference . Then I added 100ohm Virtual grounding resistors to the driver tubes and this the cured the problem. I took off the grounding resisters to the output tubes and this had no effect. The amp now sounds better but still not there yet. Tried 0.22uf and 0.33uf instead of the 0.1uf coupling Capacitors on the inputs, getting better then the Transformers and tubes arrived for my next project so work on this amp has been side tracked.
Next project is the Tim Mellow 25W OTL Tube Amplifier with the Russian 6C33C , Dam these are Big tubes.
Having some sourcing issues to get all the parts but got about 90% of what I need so far. Started making the chassis from sheet aluminum , Dam big holes for those tube bases.
If you are in Europe I have found a source for transformers , Troidy in Poland .
Sourced the Russian tubes from the Ukraine , Ironic with whats going on there at the moment, just got them before all this started. I now have Digilent Analog Discovery 2 so will Test all my amps and start a library of tests for people to reference .(Once I sort out /re jig my set up , DAM can't stop building Tube Amp's )
no earthing to main Chassis
Tim Mellow's 25W OTL Tube Amplifier
Going Slightly Different Layout
Troidy Web Site
Size comparison of the mains transformer to the A9 (got to pick up with two hands)