How to test

I just found some info I posted on it on another forum as I was looking at it in Nov 2012. Here is what I posted.
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?

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.






Re: Teisco Tube Preamp
« Reply #4 on: November 18, 2012, 07:59:48 am »
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.

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.

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.

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.

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.


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.


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?
 
Pheeeeuw!

Got to get systematic there Drags'! Fit a set of valves and make a sheet with all the bases with pin numbers on it. Then go to each pin in turn and measure the voltages.
Smudge it then post the photo with all the readings on it. Diagnosing amp problems always starts at the output but a voltage table should throw up anything bad or weird.

Dave.
 
Pheeeeuw!

Got to get systematic there Drags'! Fit a set of valves and make a sheet with all the bases with pin numbers on it. Then go to each pin in turn and measure the voltages.
Smudge it then post the photo with all the readings on it. Diagnosing amp problems always starts at the output but a voltage table should throw up anything bad or weird.

Dave.

Little difficult being professional when you're a newby. Will get around to doing this.
 
I haven't forgot, I have to dig the Teisco out and get it on the bench, I am not sure where it is even! Might be a week to a month before I get to it, still painting and caulking and trimming the five windows I put in.
 
Teisco voltage readings.
N/A= nothing attached
STC=soldered to center of the tube
all tubes are 12ax7s
 

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Righto D!

V's one to 3 look ok and V4 is surely connected as a cathode follower.
V5 however looks very weird. Can I assume all the valves were known good ones? If not try V1 in the V 5 position and measure it again.

I shall continue to look for a schematic I have my spies out! BTW any numbers or codes on the casing will be invaluable.

Dave.
 
Righto D!

V's one to 3 look ok and V4 is surely connected as a cathode follower.
V5 however looks very weird. Can I assume all the valves were known good ones? If not try V1 in the V 5 position and measure it again.

I shall continue to look for a schematic I have my spies out! BTW any numbers or codes on the casing will be invaluable.

Dave.

I will take a look see. Not sure if all valves were good, will swap them out, I also have some brandy new ones I didn't want to put in there but can. The reverb tank is no longer in there, and the depth pot for the tremolo is out, it was frozen and i was going to replace it but the ones I bought had a shaft to large for the hole, a first form me, ha! I am not sure if one of these tubes should not be a 12AV7, I read somewhere that it came with 4 12ax7s and on 12av7.
 
I am pretty sure the tubes are as follows V1 and 2, preamps for channel 1 and 2, V3=reverb, V4?, V5=tremolo, i.e. from tracing the wiring.
 
I will take a look see. Not sure if all valves were good, will swap them out, I also have some brandy new ones I didn't want to put in there but can. The reverb tank is no longer in there, and the depth pot for the tremolo is out, it was frozen and i was going to replace it but the ones I bought had a shaft to large for the hole, a first form me, ha! I am not sure if one of these tubes should not be a 12AV7, I read somewhere that it came with 4 12ax7s and on 12av7.

The 12 A VEE 7 is a very rare valve and I have never seen one in 50 years and this is the first time I knew of its existence....
http://www.philbrickarchive.org/12av7_ge.pdf

Are you sure it is not supposed to be a 12A "U" 7? A very common low mu, highish power double triode often used as a reverb driver (the two sections are usually paralleled for greater power and have a small transformer in the anode circuit.)

No, don't put any new stock in there as yet. Pre valves are very unlikely to suffer damage but since Vs 1 to 3 look fine from the voltages use those positions to test the others.

Dave.
 
it could have been a 12au7, I have one. I probably won't get back to it till this weekend, there is a small transformer connected to one of the pins on V5, but I forget which pin.
 
it could have been a 12au7, I have one. I probably won't get back to it till this weekend, there is a small transformer connected to one of the pins on V5, but I forget which pin.

Yes, far more likely a U7 (ECC82).That would also explain the voltages on V5, it is the tank driver. Did you say you had disconnected the reverb tank? If so leave V5 out.

Dave.
 
I can almost guarantee that tube three went to the reverb, the wires go from which pins I cannot remember at this time to the tank from tube three, the wires from the pots for the reverb also went to V3, the pots from the tremolo went to V5, I am almost sure of this, I will check again and get back with ya. Yes the reverb tank is out, it might as well not have been in there because it was shot.
 
Put the original output tranny back in the Masco and she works just fine but when I hook up the feedback circuit I lose about 90% of the volume.
 
Put the original output tranny back in the Masco and she works just fine but when I hook up the feedback circuit I lose about 90% of the volume.

Yes mate! THAT'S what negative feedback does! It reduces the gain. If you are going to keep the amp for guitar you could replace the 3k3 with a pot, 22k say and have variable feedback but keep the 3k3 in series with the pot otherwise you will go instable again.

Re V5. If you are not replacing the tank you could change the valve's duty to an FX loop. Merlin Blencowe has some good circuits for this.

Dave.
 
Yes mate! THAT'S what negative feedback does! It reduces the gain. If you are going to keep the amp for guitar you could replace the 3k3 with a pot, 22k say and have variable feedback but keep the 3k3 in series with the pot otherwise you will go instable again.

Re V5. If you are not replacing the tank you could change the valve's duty to an FX loop. Merlin Blencowe has some good circuits for this.

Dave.
As far as the masco is concerned, the volume coming out with the feedback circuit hooked up is so low I don't see how they would even call it an amp? Maybe something to do with the original preamp that came with it? I don't see how they could call it twenty watts when it sounded more like two. I am never going to hook it back up. Looking to get more power out of it, I replaced the bias resistor with the 330 ohm you recommended but didn't really hear a difference?
I received a new book in the mail today, "Vacuum tube guitar and bass amplifier servicing" by Zottola, looks like it might be very helpful. One book I highly recommend for the novice is "Simplified guide to electronic circuits, test procedures and troubleshooting" by Hugh Moon. Very good book with very good illustrations on where to hook up all your test equipment to take readings in an amp with scopes and meters for bias, load resistors , experiments etc, I highly recommend it and you can find it on at AbeBooks - Used Books, Rare Books, New Books & Textbooks, a great place to buy books by the way. Here is a link to the book itself, price 1 dollar, shipping $2.48
Simplified guide to electronic circuits, test procedures and troubleshooting by Hugh Moon: Parker Pub. Co 9780138100360 Hardcover - Hippo Books
I know you don't need any of these books, just mentioning them for people like myself.
I bought both of David Webers books, what a waste, he is more interested in bragging about himself then he is in teaching anything and some of the shit he spouts I believe to be urban myths.
I find books on ebay then go to abebooks and get them at a fraction of the cost everytime except once.
Bought the wife a dozen large illustrated art books for Christmas, cost me around 60 bucks.

Re V5, I don't know where i would find a reverb tank that small, never saw one like it. It is kind of like 3 small short tanks in one. 3 springs, each in its own enclosure, wired together, appx 4 iches wide, X 7 inches long. You can see the tank in the pictures I posted from the top of the amp with all the circuitry, it is on the extreme right.
 

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"As far as the masco is concerned, the volume coming out with the feedback circuit hooked up is so low I don't see how they would even call it an amp? "

You are I think confusing VOLTAGE gain for power output*.
Now that you have everything working pretty well it is time to make some objective measurements.

Without feedback, inject a 1kHz sine and get a p'out of say 10watts (8.944V across 8R. Shall we say 9volts for jazz?). You will need a hefty resistive load, 50W min, iirc you have a scope? Whack that across the load and make sure it is a clean sine wave.
Now connect the feedback R and note how much you have to increase the input signal to get back to 9 volts. The difference is the gain lost by the feedback. It should be no more than can be made up by one section of an ECC83 probably just one section of an ECC82.

The bias resistor should not change the sound. Increasing the value was to reduce the current in the output stage so reducing heat and giving the valves a potentially longer life. Because the first AC30s and their ilk ran the op valves over limit, designers ever since have slavishly copied the idea and run the bllx off the valves thinking it was the way to get "the tone". Ain't so and you CAN run EL84s and such just below data sheet maxima and still get a good sound. As you have discovered.....I should be interested to see the Va, Vg2, Vk voltages on the op valves?

Re the verb tank? There were very wee units made for the ICE industry. Pretty crap as I recall! Better off without I would say.

*Negative feedback will reduce the ABSOLUTE power output of an amp. This is because sans NFB the distortion is quite high at lowish power, say a watt and tends to increase with power output until something hits a rail.
With NFB, distortion stays low until power gets close to max then, WOOOSE! Distortion goes thru the roof. Not really a characteristic you want in a guitar amplifier!

Dave.
 
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