I built two 6T9 tube amps. The first used the GE 6T9s and I have just received four RCA 6T9s. I'm intending to tube roll between the two brands. Should there be a difference or am I going to find out that one company brand engineered and made all the 6T9s in the universe?

I have some Russian 12AX7s and RCA 12AX7s and the RCA 's are long plate so very, very different different in construction. If I tube rolled between the two 12AX7s there should be a difference due to the construction.

EDIT 4 Sept 2008: Here is the Ultra-Linear Version of the 6T9 Tube Amplifier.

The Compactron tube family (including the 6T9) were originally introduced by GE. Perhaps the RCA are just re-branded. How much do they look a like? Let us know how the listening goes.

I am still burning in the RCA 6T9s. So for it to be a fair test I need about another 20 hours on the RCAs. They look extremely similar to the GEs. They are a fraction taller. I think I have stated somewhere else that 6T9s come in two sizes. Short and not so short. There is about 1cm the difference. This could change the valve characteristic a little because there may be slight difference in heat dissipation. When I feel the RCAs are run in I will compare.

I did feel when I went from the aged GEs to the new RCAs the sound quality dropped a little but that was going from matured valves to new valves. Hardly a real test.

So are you happy with the 6T9? Have you tried to improve the circuit at all? Since those Edcor transformers have UL taps, perhaps you can try and see if you prefer that.

Hi Bill,

Mark has actually done up an article for the UL modifications. I just have not had a chance to get it on the site yet. I will get it up shortly.

I'm extremely happy with the 6T9 amp using the Spare Time Gizmo brds. There are a couple of areas where the cct. can be improved:

Use the Edcor trannies and the UL tap. You can solder the UL tap direct to pin 10 which is the screen grid and leave out or cut away the 1K resistor easy.

Upgrade the coupling and input caps to .22uf. This improves bass. Use a good quality metalized polypropylene as a minimum. I use inexpensive polycaps and they sound great. Space is limited where the caps mount but don't let this stop you.

Double the last PS cap from 100uf to 200uf and snub both PS caps (.1uf) and snub the diodes (.01uf).The snubbers for the diodes will drop into the same holes on the PCB - too easy. The extra PS cap improves bass dramatically.

I went over-rated with the power tranny (Hammond) and the Edcor OPTs are rated 15W. The amp produces 4.5W!! I also rubber mount all trannies. Keep the signal earth and mains earth separate. I only used silver based solder (5%). The amp is dead quiet. No hum or hiss-nothing.

The amp has a massive sound stage and very detailed. Extremely musical I would recommend the Fostex FE167E drivers (Gio's suggestion which I followed) to go with the amp.The combo of the 6T9 and Fostex is made in musical heaven and produces one hell of a sound!!

Hi Mark,

I looked at the circuit, but I did not see any coupling caps. Is this something you added?

Cheers

Gio, maybe I'm using the wrong term but to me in the 6T9 cct. here are input caps and one coupling or interstage cap (.01uf). Have I used the wrong term for the wrong term??

This looks like a fun project! Maybe I can make a bi-amp system and run tweeters off these.

With the Edcor trannies I got 25Hz to 20kHz +/- 3db.