Hi
I´ve just finished up on an old teisco checkmate 15 tube amp
Every cap, and a lot of resistors, jacks and sockets are replaced,and everything is in working order. Actually a nice sounding amp. Sweet tremolo!
But.... the reverb really sounds like crap.... It´s driven by a 12ax7, as shown in the schem below ( I know it´s a chekmate 20 schem, but it seems they are almost identical)
The tank is really small, there ar two tanks actually?
Can someone, with more experience than me, tell me if the tank is whats making it sound awful, or is the reverb cirquit a poor design?
I´m tempted to try another tank, but unsure if this is even possible without modifying the reverb cirquit?
Best regards
I´ve just finished up on an old teisco checkmate 15 tube amp
Every cap, and a lot of resistors, jacks and sockets are replaced,and everything is in working order. Actually a nice sounding amp. Sweet tremolo!
But.... the reverb really sounds like crap.... It´s driven by a 12ax7, as shown in the schem below ( I know it´s a chekmate 20 schem, but it seems they are almost identical)
The tank is really small, there ar two tanks actually?
Can someone, with more experience than me, tell me if the tank is whats making it sound awful, or is the reverb cirquit a poor design?
I´m tempted to try another tank, but unsure if this is even possible without modifying the reverb cirquit?
Best regards
Last edited by a moderator:
I´m not musician, and don´t know about instruments amplifiers, but the output circuit of the reverb is some strange. In my opinion, the volume pot is wrong, the output would be by the center pin of it, so the .005 must be to the 2nd pin (Cursor, wiper or center) and the out from reverb to the top of it. In this way, with the pot at the low volume out, the output from the reverb receiver is being shortcircuited.
Rgrds.
Rgrds.
It's a relatively common way of wiring volume controls, but personally I absolutely hate it, and would never do it that way.
Other Teisco amp schematics show two units, in parallel, have you tried connecting your units one at a time, in case one tank is broken? Though it's more likely a problem in the surrounding circuitry.
It's a very unusual circuit around the tank(s), and along with the symbols used in the tank, and the fact that you say it's very small make me wonder just what sort of tank it is.
Usually the send & return pickups at the ends of the spring are electromagnetic, often with low z circuitry like a cathode follower, two halves of a double triode paralleled, a matching transformer, that sort of thing. None of that here. Also, the symbol used for the transducers is just like that of a piezo element, so it may well be a piezo transducer reverb.
If that's so, then you can't use an electromagnetic type without changing the surrounding circuit, both drive and receive.
I wouldn't worry about the odd volume wiring too much, I think it'd probably work reasonably.
What sort of voltages are you seeing in the circuitry surrounding the tank? Eg the voltages on the anodes and cathodes of the 12AX7 and the 6AV6. A schematic with voltages marked on would be useful.
Last edited:
I can't say I much like the reverb circuit; shorting out the drive to the spring as reverb mute means you have all the reverb noise(including the mechanical twangs and rattles even when the thing's in bypass, and wiring the volume control up backwards like that means the frequency response changes as you turn it up, and the volume of the dry signal reduces as the reverb is increased (all right, not entirely a bad thing, in either case, but I think they did it to save on component count, not to sound better). If you want to change the spring it's important you use one with the same impedance coils (I take it those are coils not piezo driver/receivers?), otherwise it's not only going to sound very strange but change the sound of the rest of the amp.
The overall input impedance is too low for my taste (the mellow sound without much attack, or bite) and the idea of shorting out the amplifier's output transformer, either with a break contact in the jack or with that – rotary switch? (and since it removes the negative feedback the amp is going to drive all the harder), RS on the diagram, seems excessively cruel on the output tubes… I don't even like the rectifier circuit on the power supply. Basically, I don't much like this amp (even if it does sound good).
The overall input impedance is too low for my taste (the mellow sound without much attack, or bite) and the idea of shorting out the amplifier's output transformer, either with a break contact in the jack or with that – rotary switch? (and since it removes the negative feedback the amp is going to drive all the harder), RS on the diagram, seems excessively cruel on the output tubes… I don't even like the rectifier circuit on the power supply. Basically, I don't much like this amp (even if it does sound good).
Surrounding circuit and symbol used both imply piezos.
RS is not a one pole switch, but a three pole switch, which appears to be a rather odd "off-standby-on" arrangement, though I think the schematic has maybe gone haywire a bit - as shown positions 2 & 3 both have heaters and HT on, just that they light different bulbs! Shurely shome mishtake? Normally the HT should be wired to only come on at position 3, not 2 & 3, though with this pole of the switch upstream of the transformer... Bit odd this, certainly worth having a look at, can't quite see how it would do anything useful as it stands.
At this power level what's actually wrong with using a voltage doubler? Anyway, I think the OP wanted help with his reverb circuit, rather than a damning indictment of the entire amp design. Looks ok to me Bagudan.
Last edited:
The schematic shows a piezo reverb unit. I don't know this model amp, but the old SIlvertone amps used that reverb a lot, and it absolutely sounded like crap - I'd get better sound from a screen door spring, I think.
Osvaldo, if they wired the reverb output cap to the center leg of the pot like common volume controls, look what it would do to the circuit. If the reverb control were turned to zero, then that .005uf cap would thenbe across the amp volume control, and that would sap your high freqs.
The footswitch jack shorts across the drive signal, but you could also use it as an ouytput. RUn the signal at that jack to some other amp for a listen. YOu could then hear the soiund being driven into the reverb to see if it sounds good to you. I suspect it will sound OK< and the piezo is the whole problem.
I know i am spending your money, but a pan like a 4FB2C1B MIGHT work. SOme Ampeg amps used a high impedance drive for the reverb - taken off the platre of a tube through a cap, like this amp.
And the reverb output, whether piezo or dynamic, is still going to be a signal voltage, which the amp recovery stage amplifies. SO a regular pan output should work. Might have to change or eliminate the 150k resistor.
DO you have ANY reverb pan sitting around? COnnect its output to the reverb pot in place of th piezo, then either drive it from another amp, or just rub the springs to see if it would work. WHo knows.
Osvaldo, if they wired the reverb output cap to the center leg of the pot like common volume controls, look what it would do to the circuit. If the reverb control were turned to zero, then that .005uf cap would thenbe across the amp volume control, and that would sap your high freqs.
The footswitch jack shorts across the drive signal, but you could also use it as an ouytput. RUn the signal at that jack to some other amp for a listen. YOu could then hear the soiund being driven into the reverb to see if it sounds good to you. I suspect it will sound OK< and the piezo is the whole problem.
I know i am spending your money, but a pan like a 4FB2C1B MIGHT work. SOme Ampeg amps used a high impedance drive for the reverb - taken off the platre of a tube through a cap, like this amp.
And the reverb output, whether piezo or dynamic, is still going to be a signal voltage, which the amp recovery stage amplifies. SO a regular pan output should work. Might have to change or eliminate the 150k resistor.
DO you have ANY reverb pan sitting around? COnnect its output to the reverb pot in place of th piezo, then either drive it from another amp, or just rub the springs to see if it would work. WHo knows.
Thanks for all the replies.... I have been googling a bit myself in the meantime, and it seems that it is in fact a piezo-tank.... Others have tried to work around it without much succes...
Chrispenycate: I agree that the weird "standby" switch (shorting out the OT) seems silly... There is still high voltage hitting the tubes as soon as you turn on the amp. Couldn´t I just use that part of the rotary switch, for a true standby, if I made it make and break the connection of the high voltage winding?
Chrispenycate: I agree that the weird "standby" switch (shorting out the OT) seems silly... There is still high voltage hitting the tubes as soon as you turn on the amp. Couldn´t I just use that part of the rotary switch, for a true standby, if I made it make and break the connection of the high voltage winding?
Standby means making the amp silent. It is only by convention we have taken it to mean removing voltage from the tubes. That is simply one way to silence the amp. In a little amp like this, ther is certainly no danger to the tubes by leaving them powered, nor will it materially afect tube life. SOme people worry about "cathode stripping," but again, that is something for one to worry about in high powered transmitting tubes or the like, not little guitar amps.
You wil find many standby schemes. Most Peavey tube amps remove the screen voltage but not the plate voltage in the finals. Some amps lift the power tube cathodes. SOme short the opposing power tube grids together. This method is unusual, but really, the transformer winding resistance will limit the currents. If you want to rewire it for a different scheme, go ahead, but it is not necessary.
You wil find many standby schemes. Most Peavey tube amps remove the screen voltage but not the plate voltage in the finals. Some amps lift the power tube cathodes. SOme short the opposing power tube grids together. This method is unusual, but really, the transformer winding resistance will limit the currents. If you want to rewire it for a different scheme, go ahead, but it is not necessary.
Perhaps with a 200K resistor in series with this cap, the signal will not be shorted, and the pot would be used of properly way: as a voltage divider, not a voltage short circuit.
I don't think this helps the OP sort out his reverb, with which he has a problem. The connection of the pot in the reverb level control circuit is not the problem. The only thing shorted out by the existing scheme is the piezo, when the reverb is at minimum. This is not a problem.
Even if you put a 200k resistor in series with the cap, then connecting that to the wiper of the pot with the piezo feeding the track end, you'd still be putting a load varying from 1M2 down to 200k directly onto the output of the treble control, causing the reverb control to be highly interactive with the volume and treble.
There isn't a "proper" way to use a "potentiometer" (which without a calibrated scale is a misnomer anyway). It's a three terminal variable resistor. Would you tell the electrical engineers that connected two terminals of such devices in the field windings of motors for speed control for many decades that they weren't doing it "properly"?
What about their use connected as two terminal variable resistors for gain control in the mic preamps of mixing desks, again, for decades? Is that not proper?
OK, mine was a comment only. I never said the scheme is bad, I only told I personally don´t like this wiring.
This is true, only if source impedance is high, but if it is low, the pot will burn out.
Osvaldo, I think you're accidentally misquoting me:
"This is true, only if source impedance is high, but if it is low, the pot will burn out."
Is your comment only! The majority of motor speed controls did indeed use sufficiently rated variable resistors
Returning to the circuit in question, both sides of the pot see high impedances - No hay problema! I do myself sometimes see circuits where I initially think "that's not the right way to do that". I'm not always right.....
"This is true, only if source impedance is high, but if it is low, the pot will burn out."
Is your comment only! The majority of motor speed controls did indeed use sufficiently rated variable resistors
Returning to the circuit in question, both sides of the pot see high impedances - No hay problema! I do myself sometimes see circuits where I initially think "that's not the right way to do that". I'm not always right.....
Well, all wiring depends on external circumstances really. Our view of electronics is sometimes voltage-centric, we have batteries, that are ok low power constant voltage sources, we have supply mains, again, constant voltage sources. But often, with valves particularly, we're dealing with higher impedances, so we need to think in terms of signal currents sometimes. The existing reverb pot wiring is a current divider for the very high impedance piezo receiver.
So relax Osvaldo. We all get to go home in one piece. We even sometimes learn a little too. Not so bad, really.
Le espero lo mejor.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.