6AQ5 Single Ended

[gtrbryan]

I recently posted seeking ideas for a single ended 6BQ5/EL84.  Well, the little amp I bought to convert has a single ended 6AQ5 instead (just arrived).  I would think this is good, since this is a popular tube here.  Any ideas for a single ended amp design?  Very simple, only want a volume control.  I would assume I could just run the preamps into the 6BQ5 of the amp I was going to build?  The current design uses a 560 ohm resistor for cathode bias.  Any ideas about another value or is this fine?  Link to current amp schematic attached. 

http://tinypic.com/r/x26nit/6

[John]

According to Wiki and another forum

http://music-electronics-forum.com/t7427/

it's pretty much just like a 6V6, just run it at lower voltages. Nice bonus is they are cheeeeeep to buy too. It seems like the Slow Watt amp used those in one version of it too, I think those mad scientists, Tubenit and Geezer were mucking about with it. 

[Tyrannocaster]

Interesting how the very first thing the signal sees in the amp is a volume control.

I have a bunch of 6AQ5s here as well as a couple of old Heathkit single ended amps with them so I'm interested in ideas for how to use them. According to the RCA tube handbook they are just like 6V6s but their upper limit is supposed to be 250 volts; plainly they will go higher, but they are a lower voltage 6V6 in a smaller tube.

[Willabe]

I think those mad scientists, Tubenit and Geezer were mucking about with it.  

Yes, I believe your right.

And you can find the evidence in the schematics section.    

Got a few pair of NOS stock layin around for myself that I bought a few years ago.    


          Brad      

[HotBluePlates]

I recently posted seeking ideas for a single ended 6BQ5/EL84.  Well, the little amp I bought to convert has a single ended 6AQ5 instead (just arrived).  ...  Any ideas for a single ended amp design?  Very simple, only want a volume control.  I would assume I could just run the preamps into the 6BQ5 of the amp I was going to build? ...

Simple. Fender Champ. Or the preamp of whatever you had planned to use with the EL84.

... The current design uses a 560 ohm resistor for cathode bias.  Any ideas about another value or is this fine? ...

My recommendation:
You have a (hopefully) functioned single-ended 6AQ5 amp. Leave everything about its power supply and output stage as-is, including the cathode resistor. Since it does the job now, there's no sense at this point in tinkering with it. Instead, turn your attention to the preamp.

...  Link to current amp schematic attached.  ...

Interesting how the very first thing the signal sees in the amp is a volume control.

And, bad from a signal-to-noise standpoint.

The schematic shows a Volume control, then ECC82/12AU7 gain stage, then second ECC82/12AU7 gain stage, then 6AQ5.

Steps
1. The Champ plan would be to swap an ECC83/12AX7 in place of the ECC82. Plate and cathode resistor values are close-enough for now, so leave them until after you taste-test.

2. Remove R1 (Volume control), C1 (10,000pF or 0.01uF) and R2 (470k). In place of R2, use a 1M resistor. You can add a grid stopper, if you want, from the non-grounded end of the 1M resistor to pin 2 of the preamp tube (now ECC83/12AX7).

3. Find R5 (1M) between C3 and pin 7 of the preamp tube. Replace it with a 1M audio pot (your new volume control). Looking at the back of the pot, with the lugs pointing up, the left lug gets grounded, the middle lug goes to pin 7, and the right lug connects to C3 where R5 did before.

Now, play the amp as-is, and sort out any errors you made to this point. See what you think. If the amp is too-tame, find where C6 and R10 connect from the output transformer to the preamp tube pin 3 and R4 (1.2k). This is a negative feedback loop, and if you disconnect it, the amp will be a bit more raucous. The connection depends on how your amp was originally built; it might be a wire running from C6/R10, or they might physically connect to the end of R4 that goes to pin 3.

I highly recommend doing only these steps before contemplating further "mods". You may find the amp largely does what you want, and needs only minor parts values adjustment after these basic steps.

[gtrbryan]

Excellent advice from all. I will leave the power amp and power supply as is and add a Champ preamp.  I think I have enough parts lying around I won't even have to order anything.  It may be a couple weeks before work lets me get my soldering iron hot.

One other thing for you all.  This is a small "audio" amp with a single 6x9 speaker.  The speaker is 4 ohms, and I'm sure it isn't specifically designed for guitar.  Any ideas if I will need to replace it?  I may try it and just see how it goes first.  However, it looks a little "weak" for lack of a better term. 

[HotBluePlates]

Try it and see what happens. Fender made a huge number of Champs and VibroChamps with 4Ω 8" speakers.

Or you can get any two 8Ω speakers and wire them in parallel, maybe even in their own speaker cabinet.

[gtrbryan]

After getting some good advice here, I finished my conversion of the old phono amp with the single ended 6AQ5A.  I went a little different route and used the Gretsch 6150 as a guide for my preamp.  I left the power supply and power amp stock, with the exception of making a center tap on the fills with 100 ohm resistors.  A link to the final schematic I used is below if anyone is interested.

http://tinypic.com/r/350umn7/6

It makes a nice amp.  The little 6x9 speaker does better than I was anticipating.  It can be a little "trebbly" but rolling off the guitar's tone pot seems to take care of that.  Thanks to all for the good advise, as usual.

[John]

Thanks for following up and posting the schematic!

[HotBluePlates]

The little 6x9 speaker does better than I was anticipating.  It can be a little "trebbly" but rolling off the guitar's tone pot seems to take care of that.

Have you tried the amp without the 1000pF treble-bleed cap on the volume control? If you play at low volumes a lot, that will make the sound brighter.

Of course, you probably play the amp turned up enough to make that cap irrelevant. Yes, the 6x9 speaker may have been intended for a wider frequency range (especially treble) than what is typical in guitar amps.