Teisco Tube Preamp

[dragonworks]

New here so not sure where this should go. I am just wondering if anyone has ever seen one.
I have a Teisco tube preamp that I cannot find any info on or anyone else that has ever seen one. It was given to me about 35 years ago and I have been carrying around the country with me ever since because it is so cool. When I got it, it didn't work, had 5 12ax7s in it. Stereo, two channel, with reverb, tremolo, low, mid, high, eq. I have contacted Teisco Twangers site on two occasions but never received a reply. Upon further investigation I found the diodes in the PS were reversed, I reversed them and was able to get a signal through although quite distorted. The trem and verd did not work. I have done a little bit of work on it over the years but don't really know what I am doing and not being able to obtain a schematic doesn't help at all. For size reference, that is a quarter I stuck on top of it.
Anyone know anything about this beast?

[PRR]

You don't need a schematic to troubleshoot tube guitar amps.

I would start with basic voltages.

6V AC *across* the heaters?

Main power filter-caps DC voltages.

Grid, cathode, plate voltage on each section of each 12AX7 (just do one side for now).

Something may stand-out as very-unlikely.

[dragonworks]

easier said than done with a novice. I am trying to get my Masco amp going and I have a schematic for that. See picture of Sams photofact below. It is just the amp without the preamp. It was given to me a few years ago. When I fired it up one of the filter 4 in one can caps melted. I replaced both can caps, then found the coupling cap on one side of the push pull was bad and a bad 20 watt filter resister, replaced them. Now the gain pot is so bad I have to replace that and then I might find out what is wrong with the rest of the amp! I have a signal coming through, weak and I have an old tube am radio hooked up to it that I know works but the fidelity is poor. There is a weak signal coming through but hard to tell if it is distorted or not, I will have to hook up my AF generator to it which I have never done. The only thing I have done with AF generator is hook it up to my scope which I don't know how to use much and check the waveform. I am a real newbie at this.
The preamp is a transformerless power supply and I think it has a short to ground, how do I dertemine that? I don't want to touch it again until I find that out and fix it it is bad. I found that the two diodes in the power supply were wired in opposed to each other (I guess someone else was in there before me). I wired them correctly then was able to get a signal through though weak and distorted. I then replaced all the pots, most were frozen. Still weak and distorted. Then I went to hook up ground from my meter one time after that and I got an arc doing it, and I haven't had a go at it since then. I am not even sure they are the tubes that are supposed to be in it.

[PRR]

> The preamp is a transformerless power supply

Unlikey.

Can you post a picture of the _inside_?

[dragonworks]

I will try and get a picture of the inside. On further investigation, (I dragged her out and hooked it up, haven't looked at it in years)it does have transformers. One small one for the tube filaments and one small one for the power supply. (I know much more now than the last time I looked at it, which isn't much) I ran the AF generator (an old HP)into the input of channel one and started checking with the scope and saw the signal on the preamp tube and at the output. As I rotated the volume control I could see the amplification on the scope and hear it in the speaker I had hooked up to the output of the "right" channel. This seems odd but when I plugged it into the second channel I could hear an oscillation coming from the AF generator? but nothing from the speaker output. The speaker output has two outputs, right and left with a switch mono, stereo. When plugged into channel one the signal would come from both outputs regardless of where the mono stereo switch was set so I am guessing the switch is bad? Anyway, I am having a blast doing this. I looked up some reverb and tremolo circuits so I can see if I can trace a signal to and through them. Four out of the five 12ax7s are using both halves of the tube and one is using one half of the tube. I have not been into it long enough to see which is connected to what but I think the one using one half of the tube is the tremolo circuit.

[dragonworks]

here is a picture of one side that I took when trying to sell it on ebay. You have to flip it over to see the transformers, filter caps and tubes. I received a few comments on ebay that it wasn't a preamp. I don't know much about digital photography and when I view this picture on my computer it is pretty large but it shows up here pretty small. I can figure which tube is for what function by following the circuitry back to the volume controls for either channel and back to the controls for the reverb and tremolo, that leaves me with one tube that I don't know what it is for yet. There is such a maze of resistors and components that for a novice such as myself it is quite daunting.
I just found out that if you click on the picture it blows it up to the size I see on  my puter. Yay.

[sluckey]

What you have is indeed a preamp. It is not capable of driving a speaker. The outputs are line level suitable for driving a power amp. The tube lineup includes five 12AX7 tubes and a single 12AV7 tube. Your preamp is the head part of an amp set that Teisco called the King. The 2x15 speaker cab also had a BIG power amp sitting in the bottom. Here are some pics of the speaker cab with the built in power amp.

http://www.guitar-museum.com/guitar-20283-1960ES-TEISCO-DEL-REY-KING-AMPLIFIER-TUBE-TYPE-WORKING

So, don't try to connect your preamp to a speaker.

[dragonworks]

You will have to bear with me, I don't really know what I am doing. The chassis only has five tube sockets. I already plugged a speaker into the output and I could hear the AF generator in the speaker using channel one and the volume control working. I hooked up the scope again. The input for channel 1 goes directly to the grid of section 2 of the first tube. The volume control  for channel 1 goes directly to the grid of the first section. I can see the signal on the both grids and I can see the amplitude change on the control grid of section one when I operate it. I don't see any change on the grid of section2.  I can see the signal on the plate of the tube, I have to change the volt div on my scope to make it smaller to fit the screen so I am guessing? that I am seeing the amplified signal there. When I operate the volume control for channel one I see no change on the signal on the plate? It  is the same for channel 2. Now, when I am plugged into channel one I can see no signal on either grid of the 12ax7 in the reverb circuit, nor can I see a signal on either side of the reverb tank. (I have switched out tubes, I have a bunch of 12AX7s but no tester). When I am plugged into channel two, I can see the signal on the grid of section two in the reverb circuit  and not on the grid of section one, and I can see a signal on both sides of the reverb tank. Small on one side, large on the other. When I operate the depth control of the reverb I can see the signal amplitude change on the weak side but not on the large side. I can see the signal at the output but it seems much weaker and distorted compared to where I am seeing it everywhere else. The mono stereo switch on the output jacks  is working.

[dragonworks]

I am going to have to call you Mr Sluckey from now on because it seems you have finally identified it for me. After all these years and asking many a person and then asking on the internet. Here is an ad I found on google images and it certainly looks like it. I am a machinist, let me know if I can make anything for ya. Couple of things I forgot to  mention. On the one side of the reverb tank with the large signal it looks more like a sawtooth than the nice sine wave I am getting out of the tubes. In the reverb circuit it appears someone added three electrolytic caps. The reason I think this is because they look newer, they are blue, all other caps and components are grey, and they are rated at 15 volts, all the other caps that are obviously original are rated 350 or more volts regardless of their size. I am tempted to cut them out of the circuit and see whats up? Going through the resistors with my meter, checking by going to both sides of the resistors it seems I have at least one open one. It appears in the tremolo circuit there is a 250k resistor that I get an overload reading when I go across it and 250k one right after it read 250K. I am not sure about reading resistance while it is still in the circuit. I have another one that is on the transformer that feeds voltage to the plates of the trem circuit that is 100 ohms and it reads 11 ohms. I am getting 312 Volts DC on the plates of that tube and my Sylvania book states that the plate voltage for the 12AX7 should not exceed 300 volts? Maybe it is the wrong tube in there? So much to learn.

[sluckey]

Here's another ad for you. I crossed your trail several times while tracking this. You've been looking for a long time! BTW, the tube lineup is a quote from a forum I saw. I really don't know which tubes are supposed to be in it.

[dragonworks]

Thanks again Mr Sluckey. See the post above yours, I added some more info in it, that is if you still feel like giving me some tips.

[PRR]

> I am having a blast doing this.

Best way to learn.

> on my computer it is pretty large but it shows up here pretty small

As you found: the pix post small but click big. That way we can skip the boring photos, they don't waste screen space. (There is another way to post large images large, but over-doing that is rude.)

> It is not capable of driving a speaker

Literally: it will make a very tiny noise in a speaker. Enough to know it isn't dead. I do agree that it would be MUCH better to test with a Power Amp between this and the speaker. Some of the strange observations may just be strain from driving a wrong load.

> voltage for the 12AX7 should not exceed 300 volts?

Don't worry about it. 300V if you just build stuff without measuring, 360V if you check every unit you build.

You would also subtract any voltage on the cathode, but that only brings 312V to 306V or so.

In fact major makers have put 400V on 12AX7 and they run for decades.

> reverb ...added three electrolytic caps. ...rated at 15 volts.... I am tempted to cut them out of the circuit and see whats up?

*Don't*. It is very likely some caps only need to be 1V or 3V, and 15V is a common (and cheap) rating for these uses. They may be from a different factory than the 350V caps, hence the different wrap and color. The plastic may have aged better, so they look newer. Let's leave them in for now.

At a glance I 'know' the factory which built it. (The hollow dogbone resistors are the big clue. Are the rectifiers little green pills?) The stuff also sold as "Elk" and late Kustom and several other brands. (I maintained a huge Kustom for a while.) None of it is now valuable, though I suspect it is mostly better than most folks recognize. Certainly if I were messing with orphan amps today, I'd love to bring it back to glory; but I have other projects and I don't have any use for it.

[PRR]

> The preamp is a transformerless power supply and I think it has a short to ground, how do I dertemine that? I don't want to touch it again until I find that out and fix it it is bad.

What you mean is: you got a shock.

It has a 2-pin power plug.

You really should convert it to modern 3-pin. Does someone have a link to instructions?

It may still have a short. But with 3-pin, a good short from power to chassis will blow a fuse, not leave the chassis "live".

A true short is full 120V. What is more likely in that vintage is a 120V or even 60V shock through a 0.05uFd capacitor. At the time, such a shock was not considered "Lethal", though it sure could hurt. And in extreme cases, it could kill, hence the modern trend to real 3-pin power and solid grounding.

[dragonworks]

I didn't get a shock, and someone has put a three prong plug on it. It is not shorted to ground as I feared.  As far as the capcitors go, as I stated before, someone replaced the diodes and they had them wired in opposing each other. That was the first thing I fixed. I am guessing that whoever put the diodes in also put the three caps in? I am guessing that the one half of the 12ax7 for the reverb circuit is for channel one but no signal is getting there? I would just like to bring it and the masco back to life and see what they sound like. I am closer on the masco.

[six_eight]

My very first amp was a Teisco Del Rey Checkmate.  It was a 50 watt head.  It had a matching cab.  I think it was a 2 x 12.  Sorry to hijack this thread. 

[Davidg]

Dont mean to hijack either but one of my first and best sounding rigs was an old teisco checkmate 20 with a dean markley overlord tube distortion pedal and man was it sweet! Course I thought it was junk at the time and traded it in on a peavey classic vtx.And one thong no one has mentioned but you cannot check a resistor in circuit-you must at least unsolder one end and also dont try to take any resistance or continuity measurements with the amp on or u will fry your meter,if your getting something on a speaker with a preamp signal u really need to run it into a power amp of some kind before even trying to diagnose a problem,it may even work as is with some power.Good Luck and dont give up on her!

[dragonworks]

I haven't given up but I am not getting anywhere. I get lost in the maze of resistors and capacitors. One tube, I think it is the reverb tube has no signal on the grid. I can see the signal at the input, the preamp tubes, the outputs. There is something wrong with channel two, the signal gets quite distorted. Channel one seems OK. I put it through my Super Reverb. The reverb tube has signal in one half of it but nothing comes in through the grid of the other half. It seems like the tremolo circuit is working, checking with the scope I can see the oscillations and the speed and depth controls work but I am getting no tremolo. Maybe it will take me another 30 years but I don't have that long to go. I know I shouldn't plug it into the front end of my super reverb but I have not other amp to try it through. I made sure I kept it on quiet side.

[Willabe]

Have you seen the signal tracing amp building/repair tool? It's easy to make and easy to use. With it you can easily trace the signal to find where it stops or becomes noisy. A number of guys here have used it when they hit a wall and quickly found the problem with it.

Here's the link to it in Doug's library of tube amp info;

http://www.el34world.com/Hoffman/tools.htm

I haven't given up but I am not getting anywhere. I get lost in the maze of resistors and capacitors.

Road trip! Need a road map!    

You might need to draw out the amps circuit and layout so you know what's going on. It's a good thing to learn and sooner or later we all need to learn how to draw out the schemo and layout.


                  Brad      

[dragonworks]

Here is where I am so far. There are five tubes, 12ax7s were in it when I got it. Four out of the five tubes are using both sections. The first two tubes are for their respective channels and they are getting signal and passing the signal. Channel one does not use the tremolo and reverb. It goes straight out and passes a clean signal. Channel two passes a signal but no tremolo or reverb and the signal gets distorted by the time it gets to the output. At the output for channel two is a small transformer or choke?  300 or so volts from one side of it to the tube that I am pretty sure is for the tremolo. The other side goes through two resistors to the output. The other 3 tubes seem to be getting voltage everywhere they need it. In the tube I believe is for the tremolo I can see the signal on both grids and plate. On the tube that only uses one half I can see the signal and when I attenuate it with the volume control both it and the tube I think is for the tremolo show that the volume control on channel two is working. The tube that I believe is for the reverb has voltages where it needs it and I can see the signal in one half but in the other half, where the grid control should be hooked up I see no signal.
This is also the tube that has the odd capicitors hooked up to it, they don't even look from the same era. The tube is wired and there is voltage on that plate, but nothing on the front end of the amp puts any signal on the grid. I can see the same signal that I see in the one half of the tube at the input of the reverb tank (A large singal).
I can only see a very small signal on the output of the tank which goes to the depth control. Also, without any speaker hooked up I can hear the oscillation of the AF generator coming from somewhere? I have a bare 1/4 phono jack in the footswitch jack so I can open and close it like a switch and when I close it the oscillation stops. I can see the signal on the one half of the tube drop also.
I don't know how to test the reverb tank to see if it is any good? How would I go about that. From what I see on the scope not much is getting through.
Thanks for the help and the advice all. I was thinking of trying to draw the circuit but that is pretty daunting for me at this time. I haven't even drawn simple circuits much.
It seems the signal is getting passed from channel two to the tremolo circuit but I am not sure where it gets reintroduced?
It seems no signal is being passed from channel two to the tube that I believe is for the reverb through the grid control.

[punkykatt]

"I don't know how to test the reverb tank to see if it is any good? "
  Pull the tank and check for any broken wires to the RCA jacks. If no wires broken, with an ohm meter check for any resistance across the the RCA jacks. If you get no readings means the small coils are open and you need a new tank.

[sluckey]

What is the voltage level of the signal you are applying to the input of the amp?

[dragonworks]

Thanks again all.
There are no rca plugs on the tank. The tank is enclosed, about six inches long three inches wide, hard wired. It is three sections with  the input soldered to one section  on the input side then tied to the other two sections. Same on the output. There are three grounds on each side also. The output goes to the footswitch and the depth control. Shouldn't the depth control go to the grid of the tube in the reverb circuit. I am not sure exactly where the input is coming from yet? but I can see a strong signal at the input. And as I stated, without a speaker plugged into anything I can hear the oscillations of the AF generator from somewhere, maybe microphonics in the tubes of the AF generator or the preamp? As stated, it goes away when I close the reverb switch?



I can vary the voltage of the signal coming in from 0 up to where I see it clipping.
I invested in a good AF meter and am just learning how to use it. I have it set at 1000 hertzs.

[sluckey]

What is the voltage level of the signal you are applying to the input of the amp?

Measure it with your scope.

[dragonworks]

What is the voltage level of the signal you are applying to the input of the amp?

Measure it with your scope.

thanx again one and all.
at the risk of appearing stupid, which is probably to late, I don't know how to measure it with my scope. I can compare the signals and tell if is being amplified and a ratio but I just started using the scope and not sure how to relate the waveform to the graticule on the scope. I am not sure how to measure the frequency either. I have a 100MHZ tectronix? scope. I think model 1741. I can dial the frequency in etc but to relate it to the numbers on the knobs I haven't got to yet. Like I said, I am just a grasshopper here. I am sure I am seeing the injected signal where I am checking because if I increase the output of the generator or the frequency I can see the change on the scope.
I can see it clip as I increase the output of the AF generator. On channel two I can trace the signal along part of the path and on one side of one resistor the signal is clean, on the other side of the resistor it is a bit clipped regard less of where I set the amp volume or the output of the AF generator. There are so many things that could be wrong here after thirty five years of sitting idle. I replaced the reverb pot an a tone control on channel one a while back because they were froze. I should replace all the pots, it could be the reverb tank won't pass a signal and I know someone has been in there before me.
Could I bypass the reverb tank, or maybe route it through my super reverbs reverb tank?

[dragonworks]

Finally have a nice clean signal coming through the Masco. Still working on the teisco.