SE 6AQ5 stereo amp

[ThisLifeILead]

Hey everyone, first post to the forum!

I cannabalized a defunct Sony TC-500a tube reel to reel. It has a matching pair of HUGE 5K OT's, a solid PT and all the tubes to compliment. I have loads of experience with building guitar tube amps so i was thinking a small desktop tube amp would be a fun project. Designing linear output stages is a different animal so im looking for alittle guidance.

Single 12ax7 gain stage to drive each 6aq5. maybe tube rectifier. (probably ha)
attached is a file i found and think it would be an easy place to start.
Single volume control. just something very basic.

Please let me know what you guys think. I have some killer old mouldings pulled from a 100yr old building that i wanna make the chassis out of and use a black steel hammond chassis bottom plate (1431-22bk3 8" x 12") as the working area.

[Mick Bailey]

Applying some NFB to small audio amps improves the linearity and distortion no end, though at the expense of some volume loss as there isn't a great deal of excess gain to play with. I've used local feedback as per the RH84/RH807 design to good effect and this works well with broad range of tubes. I prefer this to global feedback. The sound from these small amps can be really impressive.

[guitician]

I have an old Silvertone SE stereo integrated amp that is excellent! The first time I listened to it, on my very efficient speakers, I couldn't believe the bass I was getting from it. Model 1477 I think it is. Just two 12AX7's and two 6BQ5A's I think it's simplicity is why it sounds so good. Tiny output transformers too.

[Mick Bailey]

I built a scrapbox amp using two tiny 'universal' transformers - you can make a fist around them. The chassis is just 2" deep and the transformers fit inside. I used the circuit design from a B&W TV audio stage from the 50s using just 2xECL82 and an unknown tube rectifier. I love this little amp because it performs so well. More than enough bass and a haunting 'closeness' with female vocals, though it does need efficient speakers.

That defunct Sony looks just the ticket and something I'd like to have for myself.

[guitician]

I checked the model and it's 7400, no info on them that I could find.

[ThisLifeILead]

Well then it is possible to drive the 6AQ5's with a single stage of gain :-)
i ordered some Chassis mount RCA jacks to get the ball rolling on this project.
thnx for the feedback guys! i will get some pics up when im all organized!

[Mick Bailey]

It depends what your input signal level is. A single stage will drive the tube fine off a CD player as many will put out a decent signal. Most of my amps drive the power tube (Usually 6L6, 6V6 807 or EL34) off 1/2 12AX7. I include a triode-pentode switch, though usually this gets left in pentode mode. I use a dual 50k pot as a level control on the input.

To connect to my vinyl turntable I use a DIY RIAA phono preamp. All of this works exceptionally well. I've abandoned commercial gear as I like my own stuff better.

Take a look at this design as a typical example (though I'd implement the screen supply differently)

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[ThisLifeILead]

Mick, thats alot better then the one i posted. I also agree the screen supply needs to be changed.
I have been looking at DIY RIAA preamps and i cant decide which one i will like...so many opinions on forums haha. Do you mind sharing what RIAA schematic you used? I want to build one of those also. I will probably just build it into the amp.

[Mick Bailey]

I use this design and it works really well. 2x9v batteries last a long time..... if you remember to switch it off. Very low noise and easily enough signal output to drive your amp. If you build it in you could obtain the necessary dual supplies from the tube heater supply.

RJM Audio - The Very Simple Phono Stage

[ThisLifeILead]

hmm the link doesnt seem to work. looks like the website might be down.

[guitician]

Big thread here about it...DIY phono preamp - cheap and simple! - diyAudio

[ThisLifeILead]

Allrighty, i got the reel to reel torn apart and my 16"x8" chassis is taped and labelled for drilling. Tested the PT and all is good. I plan to replace the 6AQ5 tubes with 6v6's. the 6ca4 rectifier with a NOS sylvania 5y3 and the preamp will be a single 6sl7.

1 question tho. The Output transformers have 2 secondary windings. one which is the speaker outs. 8 ohms and the second winding is labelled 600 ohms. is this for a balanced line out? what else could i use these windings for?

[Enzo]

Not much else.

[ThisLifeILead]

thnx enzo. thats what i figured. too bad its not a guitar amp haha.

[Leo_Gnardo]

the second winding is labelled 600 ohms. is this for a balanced line out? what else could i use these windings for?

There are 600 ohm headphones, & higher. They'd work a treat with your 600 ohm outs.