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Does anyone recognize this tone stack?

PostPosted: Sat May 09, 2009 10:57 am
by codamedia
This is from a Beltone (Teisco) AP-22 amp with a single 12" speaker. It is approximately 18 watts, with 2 EL-84's, cathode biased. The preamp is a series of 6VA6 Tubes including one for the tremolo.

This tone stack has a distinct tweed quality, with honky mids, and little to no bass response. I have been searching online and cannot see anything remotely similar. I am wondering if anyone has seen this in other amps, and if so which ones.

Image

Just to clarify the bad drawing:

1: BASS 1 & 2 along with Treble 3 connect to ground
2: Bass 1 & 3 are joined by a .005 cap
3: Treble 2 is output, and connects immediately to a .7nf cap
4: Treble 1 is not only the input but also joins the output with a 500k resistor (it joins output after C2)
5: Output eventually goes through a .01 cap then to one of the 6AV6 tubes.
6: Input looks normal with the 100k resistor to the bass control, but there is no cap prior to the treble input which most stacks have (250p or 270p, etc...)

SECOND QUESTION!

It does sound great, but I'm never happy - LOL! In a quest for a little more lows, I am considering replacing this tone stack with either a "Brownface" stack, or a standard FMV stack, with a fixed mid resistor. Both ideas can be viewed at this site:

http://amps.zugster.net/articles/tone-stacks

Does anyone see any immediate dangers in doing so? Any tips or suggestions?

This tone stack removes with just three solder points, so I would keep it in tact and simply install a new one. That way I can quickly go back if I need to or want to. My concern is more with how the new tone stack may interact with the preamp. Are there impedance issues I should be really concerned with?

Thanks in advance for your time.
Sean

PostPosted: Sat May 09, 2009 3:17 pm
by Geek
Hi,

The real caveat with tone stacks and AFAIK, Duncan Amps is the only one to cover it - source and load impedance.

The source and load impedance will have a dramatic effect on the tone stack's response and insertion loss.

Load impedance is usually not an issue since it's usually a tube grid or a 1Meg volume control.

Of all the stacks I've tried, a modified Baxandall has the best control, unfortunately, the highest insetion loss. It needs to be driven with a cathode follower (~5K source Z with a 12AX7).

One not listed on that page that makes a real sweet single control is the Big Muff.

The Big Muff stack is neat in that it's response is almost independent of source impedance.

Another neat trick of the Big Muff is if you place it in the amps NFB loop, you have yourself a parametric mid EQ.

Here's examples of the origional Big Muff stack:
http://www.pisotones.com/BigMuffPi/psst ... rsions.htm

Here is a Big Muff as a tone control:
http://www.ckamps.com/docs/FuzzyPup_v0.gif

And an example of it in a FB loop:
http://www.ckamps.com/ToneGeeks/ToneGee ... 166#msg166

Cheers!

PostPosted: Sat May 09, 2009 4:48 pm
by codamedia
I kind of figured there would be impedance issues, but I was figuring on the output, not the input. Thanks for that heads up!
Is there any way to calculate the source inpedence? I'm pretty good with a calculator if I know where to start :))

BTW thanks for the other links - I'll look into that option as well.

PostPosted: Sat May 09, 2009 9:45 pm
by Geek
Happy to have been of some assistance :))

codamedia wrote:Is there any way to calculate the source inpedence? I'm pretty good with a calculator if I know where to start :))


I just use the guide at the beginning of the RCA Tube Manual. It's in all of them, regardless of issue.

(you're gonna make me dig it out for the formula, aren't you? (lol) )

Cheers!

PostPosted: Sun May 10, 2009 7:13 am
by codamedia
No I won't make you dig out the calculations :))
I'm one of those that likes to be pointed in a direction and take if from there.

Thanks

PostPosted: Sun May 10, 2009 11:22 am
by crispycircuit
Input caps and coupling caps limit the bass in guitar amps. I'd increase them before I change the tone circuit....

PostPosted: Mon May 11, 2009 9:31 am
by codamedia
It did come to me over the weekend, even with the 1 meg bass pot cranked, the amp simply was not producing the low end I want to hear. It's more a low mid boxy sound.

The input caps are 103's and so are the coupling caps. I just replaced the original caps with Orange Drop 715's but kept the values the same. My high end came back wonderfully when the couplers were replaced, but the lows are still boxy.

What value's would you try in a 1-12" combo to deepen the lows?

All opinions welcome! Thanks

PostPosted: Mon May 11, 2009 4:12 pm
by crispycircuit
I'd try a couple steps larger. 103's?? Is that .01 or .1? .... If it's .01 then try a .03. You can tack caps on in parallel to increase the uF untill you find what you like..... Bottom end eats up a lot of power so your limited with small amps.... Also a super efficent speaker will make a big difference with low wattage amps.... Rock on!!

PostPosted: Mon May 11, 2009 4:24 pm
by Geek
Do you have a schematic for that? I couldn't find it at the usual guitar haunts (???)

Like Crispy says, up the coupling caps a step or two. If you have 103 (0.01uF), try a 223, 333 or a 473, whatever you have on hand.

Some guitar amps will motorboat with that much coupling cap. If it does, increase the value of the stage decoupling caps.

Cheers!

PostPosted: Mon May 11, 2009 6:57 pm
by codamedia
I've got all sorts of caps here, so I'll step it up a little as suggested. The 103 is a 0.01uF.

Before I go changing caps, I'll try some other speakers (same impedence of course) to make sure that isn't where the hold back is. I've got quite a few here to try.

Geek: No - I've looked everywhere for a schematic on this one, no luck at all. The Beltone AP-22 is a Teisco amp and from what I can see, schematics are very rare on any of these.

All in all it's a great sounding amp. 2 x EL84's, cathode bias, fair sized box and a 12" speaker. Very similar to an AC15. Since it a non-collectable, it makes for a great amp to try a new value here or there to squeek out a little more personalization :)) Thanks again for the input - I'm loving these forums.