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 )
Keep Safe
Will
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)
The Parts
Tim Mellow - 25W OTL Tube Amplifier Build / Progress
Just got some more kit that I may use for this Amp.
All holes for Tube bases Done.
Chassis now formed using my home made sheet metal bender.
Seen this Double VU Meter with driver board on Ebay , ooh, thought I'll incorporate it in the design. Original design had 2 x 1Amp meter's for biasing only, no VU Meter's. I have read in some of the forums these meter's were replaced by resistors and not fitted after initial set up. I'm going to use a switch were I can switch between the resistors and meters so as to check the biasing now and again when needed. Trying to fit the 1 Amp meters any where on the chassis just did not look aesthetically right.
Hole cutout for the Meter, took about an hour to complete , took extra care not to mark the chassis as this will be in full view when complete.
Deciding were the I/P'S and O/P'S are going.
Decided on this layout in the end.
I ordered a pair of these with driver board some time ago
to go with the Boyuurange A09
No lamp adjustment with this PCB
Dam big holes to drill thorough the face plate.
Made the mistake of measuring the resistance of all the meters and testing their deflection. 2 Pairs and I Double all varying resistances and deflections with the same current.The only consistency with parts from China is the Inconsistency. Best use these as "Indication Only" as they look like 20-30% variation between then. Having in the past calibrated Medical equipment these would be a nightmare to calibrate correctly and would be scraped.
Tested using another Driver PCB with Lamp adjustment
Low level adjustment
High level adjustment
Double vu meter Illuminated
Added some tape to make them look better
Take Care and have fun.
Will.
Hi roblund7,
Just had a 7 day holiday away from the UK in the sunshine, Looking at the circuit diagram I have noticed that the pin out's for the 6C33C on the diagram don't match the Data Sheet. Oh here we go, just at that point of deciding where to physically mounting parts things go astray. Looking at other post of people that have built this Amp we will go with the Data sheet pin outs. Now the decision to make , Point to Point wire or use Tag Board to make it look better inside. So far leaning towards Point to Point wiring as I would need to work out all the connections and component locations before building. Just itching to get started . If this Amplifier is as good as some people have said I'll just have to build another one for the house and use Tag Board for that one.
One issue so far with components is the 20 turn pots cant seem to source these yet.
I will take Photo's during the build and post here for reference and any issues I come across during the build.
Stay Safe
Will
Looking forward to watching the progress!