Good Trem circuit & a tale of a Teisco

[rfgordon]

First, like a complete moron, I neglected to take pics of this project, which was really, really dodo-headed.

Anyway, one of my guys bought a Teisco Checkmate 25 (head w/ 1/15 cab)--mostly in order to keep another one of my clients from buying it!

It worked--mostly--but he became concerned when he got a nasty mic bite at a gig. It also got icky sounding as the gig went on. So it was that it came to me. Of course, it would be the one Checkmate with no available schematic! It's basically the Checkmate 20 in fixed bias and one tube change.

The seller had told the music store (it was on consignment) that it had been serviced not too long ago. Well, that really meant a pair of Sovtek EL84s and nothing more, cuz the thing was bone stock--right down to the Japanese 12AX7s.

Issues:
1. Bias all out of whack--red plating
2. ALL signal caps leaked--yucky sounding gain stages
3. Trem no work--more on that later
4. Leaking ON/STANDBY rotary switch--it if was plugged into the wall, the filaments were glowing. What's "totally unsafe" in Japanese?
5. Bogus STANDBY mode--it simply grounded out the OT.
6. Leaky switch put half wall voltage on the chassis. He's damn lucky to be alive!
7. 1965 filter caps

He wanted it converted to cathode bias, which presented a bit of a challenge, since it had a ginorous B+: 425 VDC (The power supply uses a voltage doubler, as you can see in the Checkmate 20 schematic below.) So I ended up using a 330 Ohm Rk shared w/ 100 mF cap. It seemed to operate happily that way. This yielded about 18 VDC on each cathode, so the difference between B+ and that put the plate voltage a wee bit closer to normal max value. (Although the Peavey Classic 50 puts even more than that on the plates, so what is max, really?)

I replaced all the signal caps with M150s and a couple of Sozos. That got all the gain stages stable. Rebuilt the PS w/ new caps. The OT had an unused secondary tap, so I rewired the extension jack as a second value.

Replaced the power tube sockets and added 1k5 5w to the screens.

You can see that the Checkmate 20 uses a 6AV6 single triode for the trem. The Ch. 25 uses one side of a 12AX7 with the other unused. I looked around for a nice trem circuit, and chose the trem oscillator of the Torevibe from the Weber site. The only difference I made was changing the 25k pot to 250. I put the wiggle point on the cathode of the next-to-PI gain stage. This trem is very nice and "throbby" not choppy, with a great range of speed and depth. I highly recommend this design.

The Checkmate 25 has a cool feature--at Standby one light comes on, then at ON, that light goes off and a second light comes on. The chassis is only 1 1/2 inches deep, so not much room. I found a rotary switch at Mouser that fit for size and function. I replaced the two 6 v lamps with 120 vac neons--to simplify switching. I also switched one leg of the PT secondary for a "real" standby switch. Of course, I put on the 3-prong cord.

The head is small, so heat is an issue. I stripped out all the death caps and changed the polarity switch to on/off for the courtesy outlet. I made a fan rig that hangs off the back of the amp to pass some air thru the head.

Swapped the 1AU7 last gain stage/concertina PI for a 12AX7 for more grunt.

The reverb on these amps has to be the worst in history, with a wimpy circuit and a very strange two-part tank. Sounds funky, but he wanted me to leave that part alone. I will say it sounded a wee bit better with new signal caps.

When it was all said and done, it turned into a really great amp with wonderful overdrive that suits his small/medium club gigs perfectly.

I guess it's another case where an overlooked "budget" amp of yesteryear makes for great raw material to work with.

[BobW]

Overall it's a cool looking circuit, but a capacitively coupled reverb just doesn't have the fullness of a transformer coupled driver. If you modified the ckt to a fender type ckt, or single tube reverb like Gary's (Glasman) ckt in the Dumble section the reverb would sound much better.

[rfgordon]

BobW

Agree on all fronts for the verb. However, the client didn't want to spend money on the reverb mods (as it was, I had to charge him about what he paid for the amp for the work I did), and the space inside the Checkmate 25 head is so limited, I'm not sure I could have gotten even the short, 9 inch tank in there.

Of course, the alternative is to run that reverb 12AX7 into an fx loop, and sell the client a stand-alone reverb unit to use with the amp!

[amoebarabbit]

Hi Everyone,
I bought recently Teisco Checkmate 25, and You Gentlemen seem to be the only source of knowledge in the whole internet about this amplifier.
From what I've already learned caps need to be exchanged, tremolo doesn't work, tubes condition - probably not so good.
I'm a musician, not much of an electrician, i now i won't spoil things if i substitute parts with good ones of same value, i can do some works based on schematic, but i don't want to experiment.
I've found some old schematic, which i compared with the amp. I've redrawn original schematic and also, prepared a modified one based on what i have found in the amp.
I'd like to know what is the impact from those changes, I hope You'll be able to advise, if they are ok, or should i revert them to original.
I'll also did some photos of amp guts - attached below.

those mods are:

- 100k added between reverb spring output and ground
- death cap removed from the ground switch - nice - which is now a power switch, rotary OFF/Standby/ON switch is now configured as On/Standby/On switch, on standby it still couple O.T. to the ground.
- two caps 0.022uF (earlier in primary circuit of P.T., one served as a death cap) were moved and paired in parallel with diodes SW-05C.


Best regards,
Tom

[Phil_S]

Swapped the 1AU7 last gain stage/concertina PI for a 12AX7 for more grunt.

Whew, that was interesting and I'd think very satisfying. I wanted to ask a bit more about the PI change. I'm not wanting to argue with success here, just a point of curiosity and possible discussion.

As I understand it, the concertina is more about current than voltage. (Voltage output is uneven, and current is about the same on each half.) On this basis, it seems to me that a 12AU7 or 12AT7 would be a better choice than a 12AX7. The X is much more of a voltage amplifier. (Maybe not said so well, please excuse me.) I wonder if the 12AU7 that came with the amp was simply worn out. Finally, to the question: did you try a replacement with a known good 12AU7? If you did, what was it that sent you to the 12AX7?

[TUBEDUDE]

Looks like a lot of naked leads around that tranny with voltage on them hovering near that metal chassis. Heat shrink or spaghetti might be prudent.