Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: big-ez on November 20, 2023, 10:33:13 am

Title: Adding a 7199 channel to a spitfire clone
Post by: big-ez on November 20, 2023, 10:33:13 am: Hi all, I have a homebuilt Spitfire clone that works great. I have an extra tube socket, and I just acquired a ton of NOS 7199 tubes, AND I need a holiday project. I was thinking of using a 7199 to add a second preamp channel to the amp. The issue is that I need the two channels to share the same vol/tone stack due to lack of room on the front panel.

For the 7199, I want to use a version (without the 'squish' control) of the AX84 Blues Preamp (see attached).

Will the attached schematic work? Do I need any mixing resistors where the 12AX7 and 7199 outputs meet? Thanks in advance!
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: sluckey on November 20, 2023, 10:48:58 am: You do need two mixing resistors. 220K is a popular value. If you use a 3 position switch such as C&K 7211 (ON-ON-ON) switch you can select either channel or both (center position). Wiring is the same as a regular DPDT switch. See this schematic for an example...

http://sluckeyamps.com/phoenix/phoenix.pdf
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: big-ez on November 20, 2023, 12:12:58 pm: OK, thank you, sluckey. I appreciate it. I think with my schematic, the two signals will end up out of phase with each other, so having both engaged might not work.
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: PRR on November 20, 2023, 12:49:31 pm: True NOS 7199s are incredibly valuable to hi-fi folks.
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: sluckey on November 20, 2023, 01:31:19 pm: Quote from: big-ez on November 20, 2023, 12:12:58 pm
I think with my schematic, the two signals will end up out of phase with each other
No, they will be in phase. Each channel has one inverting stage. The CF is NOT inverting.
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: DummyLoad on November 21, 2023, 01:45:22 am: 6AN8 sound better in guitar amps + are lot less expensive - Flip those 7199 to Dynaco owners.

--Pete
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: kagliostro on November 21, 2023, 01:59:37 am: Don't know where you find those tubes but know you were really very very lucky

Franco
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: Carlsoti on November 21, 2023, 08:51:40 pm: I'd love to fall into a pile of those to play with. It'd be cool to have a few spares for my Sunn, and just to see what else can be done with them. I've always had an odd fascination with tubes with dis-similar halves. They all got so expensive before I got hip to them, though. :BangHead:
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: ALBATROS1234 on November 30, 2023, 10:07:03 am: Try a 6U8a, 6EA8 or 6GH8 much cheaper and a very similar tube to the 7199 with a different pinout. I built a SE amp with a 6U8 and a tube drive they sound really good. I will sell you a 7199 for $50 if you want one though. That's what a NOS one goes for sometimes LOL.
Title: Re: Adding a 7199 channel to a spitfire clone
Post by: big-ez on December 12, 2023, 11:30:38 am: I just wanted to follow up here. I added the mixing resistors (220K) and the new channel worked great out of the gate. It did have a LOT more gain than the other channel, which just used 1/2 of a 12AX7. I ended up paralleling the 12aX7, and on the 7199 side, I dropped the value of the plate resistor to 100K (from 220K). While the 7199 side is now still louder than the 12AX7, it's not insanely louder. Thanks for all the help with this!