Resurrecting old Teisco Checkmate 60

[Lee_ranaldo]

Hey all. i've been working on this amp for months and can't get it right yet.

Its an old teisco checkmate 60. Originally it came with EL34 although the original amp used 6CA7

The owner re-did the phase splitter so i have no idea how the original looks like. The lack of documentation
available on the internet about these Teisco amps makes me feel absolutely frustated.

I replaced the PI section with the one shown on the Checkmate 50 circuit, which is a classic long tail pair
phase splitter. Replaced the Grid Resistors on both tubes to 5.6K instead of 3.3k shown on the schematic
of the Checkmate 50 that looks very similar to the power stage of the Checkmate 60.

This unit has a tremolo with an optocoupler , it looks really like the one on the checkmate 50 and a lot like the silvertone 1484 in some ways.

Since this amp is totally third/fourth hand there is a lot of mods in it.

What i've done so far is replace the majority of caps including coupling caps and filters. Replaced every resistor (originals were carbon resistors) with 1% metal film and carbon film
for the plate resistors. Screen resistors were increased from 470ohms to 1K ohms


The pre-amp has a HUGE hiss and hum. I did the tricks of installing 100 ohm to ground resistors from the heater taps to ground to reduce hum , which worked nicely but still
got a HUGE amount of white noise, even when all the controls are off.


I wonder if installing grid resistors (33K to 68K?) on every pre-amp tube stage will reduce this symptom.

The amp originally used a tube rectifier. with 100ufx450V filter caps (5AR4 tube)..which is kinda weird.
They converted it to a full wave rectifier which i improved a bit.

There is a weird arrangement on the bias and bias splitter resistor section which does affect the overal frecuency response of the amp



I haven't been able to find the original schematic so i can restore the original pre-amp. I'll attach the schematic for the checkmate 50 . They are quite close
except for the "bright" switch arrangement.

Any info/tips would be appreciated in this build. I've lost a lot of hair doing some design, trying everything i have on hand to reduce hum and noise.

[Enzo]

Forgetting the hiss, how does it sound? If it sounds good and strong, but also hissy, that is one thing, but if it sounds kinda thin and weak with hissy, that might be another - RF oscillation perhaps.

Adding resistors to all stages? The first response to a problem in an amp should not be redesigning the thing. FIND the hiss and cure it, THEN do your redesigning.

SO all your controls are at zero and it still hisses? OK, there is a lot of amp after the controls. You have two channels, so pull the tubes from one channel at a time to see if one or the other is contributing, or if both do equally. You can also ground off the signal path selectively, after all the hiss is riding the signal path. Many points on the signal path have DC voltage from ground, the plates of your preamp tubes for example. SO look in your parts, and grab a 600v film cap - I use .047uf myself, just because I have a bunch - clip one end to chassis, and another wire clipped to the free end of the cap. Now use this wire to "ground" points in the circuit to see if they kill the hiss. The cap will block any DC, but hiss signal will be shunted to ground through that cap. Even if it doesn't kill the hiss, if it drops the amount, we know we are on it. BY stepping along the signal path we can find th point at which the hiss enters the stream.

[Lee_ranaldo]

Well. it sounds loud but hissy. Its "playable" but still a lot of hiss going on. I have a 2x12 cab with whaferdale speakers
so i can hear every nuance of hiss above 6K-8khz easily.

The weird "bright switch" array is totally weird. it basically feeds 2 outputs that come from the first
triode from the first pre-amp tube on each channel together through a capacitor.

I'll get everything traced down correctly and post it here. This arrangement looks totally weird
to me. Maybe i'll make more sense when i have a chance to complete the tracing of the whole pre
amp.

[Lee_ranaldo]

i've traced the pre-amp partially.

Reverb signals and tremolo circuits are not included yet.

On the checkmate 50 schematic some 33uf cathode capacitors are not included.T he checkmate 50 doesn't include an extra triode stage buffering the signal
that goes to the phase inverter. On "Channel 2" there are 2 triode stages with no grid resistors between them. I am thinking testing it out with Enzo's method and then probably remove a couple of Cathode bypass capacitors. Including a grid resistor and check how that works.

The amp is wired with Shielded cables between controls and input jacks. One of the shield's ends is grounded, the other isn't.

[Lee_ranaldo]

The hiss comes from the last Mixer stage.

I've used shielded wire which reduced the hiss a bit. mostly RF and the 60hz Hum induced creating harmonics and such
Second. I totally redid the ground scheme for this amp.
That removed a lot of hum and noise

I'll add 1000pf caps on each plate resistor. Since i couldn't find metal film resistors that could stand the plate voltage
i added 2W carbon film resistors.

The amp now is quieter with hum that is barely audible on humbuckers , quite noticeable on singles which i consider "OK-ish"

I Changed one of the 12AX7's with a ECC82 to reduce gain and hiss without altering the low end much.

The tubes have a huge low end response due to the 33uf cathode bypass cap. If i reduce this value the amp ends up being to "trebbly". I am
using some 5881/6l6GC tubes from sovtek to test the amp since the original EL34's are totally cooked (they make synth like sounds now).

The hiss remains constant with every volume control down until i start cranking it up. Is not dramatical and considering
this amp has a lot of gain stages hiss will occur and there is nothing you can do about it, except hitting the standby switch

I am gonna test some 15-47 pf caps connected from grid to ground on every pre-amp stage hoping i can reduce the hiss a bit more.

Since this amp originally had a tube rectifier, that was later replaced by a silicon rectifier i am guessing the switching noise is contributing to the major
hiss.

[vetter]

If you are interested I have the original schematic (it was in fact stuck on the bottom metal plate what a chance) PM me if you want

[slyraccoon]

Original Checkmate 60 schematic

I know it's probably way too late, but if anyone is interested I recently picked up a checkmate 60 all tube amp it had the original schematic glued to the inside of the bottom plate.
I took a photo of it and cleaned it up so it is more readable. I will post it here in case anyone is interested in it. I am in the process of cleaning and recapping the amp. So far I've got sound, but no tremolo. The reverb tank appears to be functional but no reverb in the mix yet. May be a weak tube or a bad cap. It's going to be a nice amp when it's finished.