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does my proposed amp sound ok?

:

2/20/1999 5:00 PM

does my proposed amp sound ok?

i have a few questions, probably basic.

i'm wanting to make myself an amp. i want reverb in it. problem is, i can only find one full amp schematic that has a reverb section. this is the yorkville ysr-1 custom reverb. i like the idea of the tremolo section, does anyone know if it is worth putting in? and does anyone know of any more schematics?

i was thinking of replacing the yorkville preamp with a fender tweed or something like that - is that a good idea? does my proposed amp sound ok to anyone?

i also have some general questions for anyone who might want to answer.

why is there a choke in most power circuits? is this a filter element? i've only ever seen them referred to as 'choke', if at all, so what value should it be (or doesn't it matter?)

in the fender tweed preamp, as in a few others i've seen, there is a twin 12ax7 section with the second one's grid coupled directly to the first one's plate, and the signal taken from the cathode. what is this called, and what is it for? does it sound good?

anyway, thanks for your help.

Rob

2/22/1999 8:24 AM
Don Symes	Plenty of other schematics here and at sites Ampage links to.

2/26/1999 7:53 AM

Rob :

One schematic I had some success with was to use the design of the Princeton Reverb preamp and reverb circuit. I then grafted on the little single 6V6 single-ended Champ power section instead of a push-pull power section, but anyway you could do either one.

There are schematics in Bruce Weber's Guide to Hip Guitar Amps, and The Tube Amp Book by Aspen Pttman too. These books are of somewhat dubious value IMHO but they have a lot of schematics in them.

A choke is an inductor that indeed helps filter the rectified AC into smooth (ripple-free) DC. Typical values in a guitar amp would be 8 to 16 Henries at whatever the current draw you need is. (Choke are rated by their inductance in Henries, H, at a specified current, usually im millAmps, mA.)

It can be replaced by a resistor but a choke does a better job of filtering out the ripple. (The slope of the filter created by a choke and the following cap is steeper than the slope of an R-C filter, so it has more effectiveness at the low frequencies which are of interest in a power supply filter.)

The 12AX7 arrangment you describe is known as a cathode follower (actually, only the 2nd section is the cathode follower) and while that 2nd section cannot add any gain, it serves to lower the output impedance of the stage. It's used in guitar amps to reduce the amount that the subsequent tone controls load down the preamp section. Does it sound good? Well, it's in the classic tweed "Bassman" preamp, which is generally held in pretty high regard.

-j

2/26/1999 10:17 AM
MJ Harnish	[snip..] There are schematics in Bruce Weber's Guide to Hip Guitar Not that it makes a whole lot of difference but it's Gerald Weber, not Bruce. :O)

2/26/1999 12:08 PM
J Epstein	You mean you haven't seen that coffee table book of naked Marshall Stacks entwined with snakes? Oops. -j

2/27/1999 9:57 PM
Rob	thanks a lot guys. about the inductors, do you mean an inductance of 8 to 16 henries or milli henries? i've never seen one bigger than half a henry. does the value actually matter very much? thanks again, Rob

3/1/1999 8:16 AM

Power supply inductors are big things, usually the size of small-to-medium output transformers. They can be open framed, or potted, or with end bells, just like transformers. Values are in whole Henries, yes.

Something little, mH, like you see in electronics catalogs, are not suitable, and yes, the value matters as an order of magnitude thing. The difference between 8 and 16? Well, more is better but a lowish one will still work - you would need to up the following capacitor value to maintain a known filter characteristic. (1/2 the L needs 2 x the C, like that. This assumes you know the value of the originally spec'd choke which in practice I never seem to.) You may want to look at the Hammond inductors on the Angela instruments web site (on the Hammond Transformers page) to get an idea of what's available for tube amp use.

www.angela.com

In general, more inductance, higher current handling, and lower DC resistance are all desirable, and all increase the price and size of the choke.

-j

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