Ahh yes... I was also watching that listing.
Just curious to see what it sold for.

I already have two so I don't need another one. 

Here's some pics of my "Blackface" version along with the schematic that matches this version.

I did change a few things... notably the tone control.
Sorry, I didn't document the changes but I used ideas from the AMZ notch tone control.

http://www.muzique.com/lab/notch.htm

Modding the tone control makes a significant difference to the tone of the amp.
For the better IMHO.

Having a little "scooped mid" helps for less muddy tone. Esp with distortion pedals.

I have seen the Surfy Bear reverb and if it could be incorporated into this amp
I think it may go a long way to improving the reverb tone.

Of the differences between this and my other "Silverface" version,
the input jack circuitry, reverb is transformer driven, and the different output devices.

As with all these amps, I added a 3 wire grounded cord and clipped the death cap.

QuoteMine looks alike (outside & inside) to your pics on page 1 at first I thought, but the placement is actually drastically different.
The sping tank is directly behind the front panel and the power devices are mounted on the base of the unit. The rear jack panel location is also different. Perhaps this is exactly describing your other unit ;-)

I don't recall the layout of the other amp.

Easy enough for me to open it up and get some pics.
I'll do that later. 

Cheers!

QuoteJust curious, how 'wet' can the reverb get at maximum?

It can get pretty wet, but it doesn't sound good. 
I'm sure this is due to the limitation of the small tank.
It also doesn't help that there is only 1 spring.

I played with part values in mine and got a "fair" sounding reverb,
but it will never be a Fender Twin.

If you "over do it" with driving the tank too hard,
or increasing the gain on the recovery side, bad noises happen.
Screeching/feedback/metallic "clank" sound, etc.

Changing some cap values and possibly adding some caps in strategic places can smooth out the sound.
Limiting the frequency range that is fed to the tank helps. 

There are other small multi spring tanks around that would fit the chassis.
I never tried any, and there may be impedance matching problems with some of them.
No guarantees they will sound any better than the existing tank.

 

Quote from: clintrubber on November 01, 2017, 09:10:23 AM

Quote from: clintrubber on November 01, 2017, 09:05:23 AM
Was wondering, any info on the reverb tank they used ? Input- & output-impedances ?

Oops, apologies, just saw the 225R & 222R indications in the last version of the schematic. Are these as measured with a DMM ? So the R_DC values ?

Will check as well on my unit.

Bye/thanks

Thanks, glad this is of some use. 

That's correct, DC resistance values measured with a DMM.

Yup, they are kinda cool little amps.  Actually sound pretty good for what they are.

Be aware there are at least 2 different versions of this amp,
both have the same name (National) and model number (GA920P) but different circuit boards/schematics.

I have one of each but not posted anything about the "other one" yet.

Well, I'm calling this one a success. 

Here 'tis all back in its cabinet.

On to the next....

Well... I found the noise! 

One of the germanium transistors in the preamp was wired backwards.
Hidden behind the heat sink.

Swapped collector and emitter leads and now just normal junction hiss.

Disconnecting various parts of the circuit helped track it down.
Looked like it had been that way since day one.

This amp must have been noisy since new.

Did folks expect these to be noisy and just live with it?

Turning up and playing, the noise was not audible so....?

Forgot to take pics and now the heat sink is back in place, oh well.

The new caps came today.
I love DigiKey, stuff comes overnight.  :dbtu:

Old and new caps from left to right.
1600uf 64V   2200uf 100V
1250uf 40V   1500uf 200V
1250uf 25V  1500uf 200V

The noise I mentioned before is about half as much with the new caps.

To describe it again, its as if you were constantly quietly balling up a piece of paper.
Kind of a unsteady hissing rustling noise.

I suspect what I'm hearing is leaky germanium transistors.

Playing with the controls has no effect on the noise.
Inputs shorted to ground and all controls at "0" noise is still there,
so I further suspect it does not originate in any of the preamp circuits.

The amp seems to work fine otherwise.

Thoughts?
Thanks!

The bad reverb sound was bothering me a bit,
so I visually traced the recovery circuit and added a 100nF cap from emitter to base on the recovery amp transistor.

HUGE improvement. 

The reverb control is bottom left in the pic.
Green cap is obviously the one I added.

Reverb is actually usable now.

Edit; In the first pic the transistor is hidden behind the cap.

Another Frankenamp monster complete. 

I picked this up about a year ago. Finally got to looking at it.

Made by Mirtone in Weston Ontario, Canada.
Info on these seems to be non-existent. No schematics, nada.
Only thing I found was some other person looking for info too.

The only date code I found so-far seems to be 1966, so mid 60's design.
All germanium negative supply. -46V rail.
It is quite loud tho I doubt 100 watts.

Does not seem to have anything in common with the USA made Silvertone 100.
Those have a printed circuit board, this one, not at all.

Has nice compression/crunch at full volume.
The reverb sounds like ass. Very artificial metallic clank noise.

Did the usual 3 wire cord and clipped death cap.
Added a fuse holder as there seemed to never have been one!

It's quite noisy (hissing/rushing noise) but I'm waiting for the 3 can caps to see it that helps.
Temp jumpered up a big cap to the main cap can and the noise got a little less, so we'll see.

Already replaced all other small electros, what a pain point to point terminal strip wiring is!
Very robust construction but a dog for repairs and making changes,
unless you just clip the part and tack on a new one. Not my idea of the right way to do it.

The Mirtone model # is hard to read, says GA-100.

Reverb tank held with failing double sticky tape so I bolted it in with some rubber grommets.

Update, I installed multi turn trimmers for the bias.
This made setting bias very easy.

Set it up slightly hot and got the amp good and warm (hot!)
then while still hot, backed it down to 25mV across each resistor like it says on the schematic.

This seems to work well.
Starting the amp from cold and monitoring the voltage across each resistor,
it starts at ~ 18mV and gradually drifts up to ~ 22-24mV as the amp warms up.
Never gets hot, just slightly warm.

Moving on with the project...

Got input jacks, AC line and fuse holder, switch and pilot light.

Kinda wondering where in the heck I'm gonna mount the reverb tank.
I did the only thing I could think of.
Mount it right on the circuit board.  :loco

Wow! this is going to be tight fit!  :duh

More pics...

You will notice that I had to "reverse" the circuit board as the input jacks are on the "other" side.

Think I'm gonna need some heat sinks to put the Gibson guts in the Sears amp.

Found some 1/4" thick copper bar that should work.
Sears amp chassis stripped down.