Reverb stinks! Tank or cirquit? - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th April 2012, 02:58 PM   #1
diyAudio Member
 
Join Date: May 2010
Default Reverb stinks! Tank or cirquit?

Hi
Iīve 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.... Itīs driven by a 12ax7, as shown in the schem below ( I know itīs 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?

Iīm tempted to try another tank, but unsure if this is even possible without modifying the reverb cirquit?

Best regards
  Reply With Quote
Old 25th April 2012, 02:59 PM   #2
diyAudio Member
 
Join Date: May 2010
Click the image to open in full size.
  Reply With Quote
Old 25th April 2012, 04:08 PM   #3
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Iīm not musician, and donīt 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.
__________________
LW1DSE
  Reply With Quote
Old 25th April 2012, 07:01 PM   #4
diyAudio Member
 
Join Date: Jul 2005
Location: North Derbyshire
Quote:
Originally Posted by Osvaldo de Banfield View Post
Iīm not musician, and donīt 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.
It's a relatively common way of wiring volume controls, but personally I absolutely hate it, and would never do it that way.
__________________
Nigel Goodwin
  Reply With Quote
Old 25th April 2012, 08:25 PM   #5
Simon B is offline Simon B  United Kingdom
diyAudio Member
 
Join Date: Dec 2011
Location: North-East England
Quote:
Originally Posted by bagudan View Post
....The tank is really small, there are two tanks actually?


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.
__________________
Engineer: One who can do for 10 shillings what any fool can do for 10 pounds.

Last edited by Simon B; 25th April 2012 at 08:28 PM.
  Reply With Quote
Old 25th April 2012, 08:43 PM   #6
diyAudio Member
 
chrispenycate's Avatar
 
Join Date: Aug 2006
Location: Geneva, Switzerland
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).
  Reply With Quote
Old 25th April 2012, 09:22 PM   #7
Simon B is offline Simon B  United Kingdom
diyAudio Member
 
Join Date: Dec 2011
Location: North-East England
Quote:
Originally Posted by chrispenycate View Post
........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?)
Surrounding circuit and symbol used both imply piezos.


Quote:
Originally Posted by chrispenycate View Post
.......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…

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.


Quote:
Originally Posted by chrispenycate View Post
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).

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.


__________________
Engineer: One who can do for 10 shillings what any fool can do for 10 pounds.

Last edited by Simon B; 25th April 2012 at 09:44 PM.
  Reply With Quote
Old 25th April 2012, 11:40 PM   #8
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote
Old 26th April 2012, 06:50 AM   #9
diyAudio Member
 
Join Date: May 2010
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. Couldnīt 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?
  Reply With Quote
Old 26th April 2012, 07:14 AM   #10
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Reverb Tank wdcw Instruments and Amps 10 28th July 2010 02:27 AM
Reverb tank hum rem280 Instruments and Amps 9 20th August 2009 06:03 PM
Gibson GSS-50 Reverb Tank question Louie7 Solid State 0 29th May 2009 10:22 PM
Reverb tank input impedance? Artie Everything Else 1 23rd May 2009 02:22 AM
reverb tank in a guitar amp connection dud_wity Solid State 1 19th June 2004 12:58 PM


New To Site? Need Help?

All times are GMT. The time now is 04:20 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2