ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

adding reverb with junkbox parts

1/20/2000 8:34 PM
Wild Bill	adding reverb with junkbox parts I know this has been flogged to death before but with no archives I hope some folks will be good enough to give me some ideas. I built an amp using the Marshall master vol preamp from the schematic on this site. Now I'd like to add reverb but this schematic has no loop for this. I've got lots of 12AU7/12AX7's etc and for a pentode I've got lots of 6AQ5's, 6V6's and 6W6's. I thought of picking off the signal after the 1st 12AX7 and using the cheap and dirty 3m3/10 pf combo and then continuing on to the existing 3rd triode stage and CF before the tone stack. I guess I'd also have to use 2 triode sections after the reverb springs to get enough gain with an even number of sections to keep the phase correct when it mixes back in. I've got some small xfmrs that are good for 3-4 watts, 2500 and 3000 ohms to 8 ohm sec. I like the 6AQ5 because they're nice and small. I'll have to add an extra fil xfmr anyway but I'd like to keep the current down to have a 12vdc fan @ 300ma hung on the unregulated dc side of an LM317 (about 10vdc across the filter cap). Any advice on which way to go or problems to avoid? Is there any reason to use one tube over another other than size or fil draw? Do I HAVE to buy/scrounge a higher value primary plate OT than 3000 ohms? Please understand that I'm not trying to exactly clone a Fender 6G15, just do it in the same style and get a decent tone with only one reverb control pot on my crowded panel. Also, I've got room for a full length Accutronics delay but will the medium length sound better to a 47 year old man who never wants to move on past SRV, BB King and George Thorogood?

1/20/2000 8:34 PM

Wild Bill

adding reverb with junkbox parts

I know this has been flogged to death before but with no archives I hope some folks will be good enough to give me some ideas.
I built an amp using the Marshall master vol preamp from the schematic on this site. Now I'd like to add reverb but this schematic has no loop for this.
I've got lots of 12AU7/12AX7's etc and for a pentode I've got lots of 6AQ5's, 6V6's and 6W6's. I thought of picking off the signal after the 1st 12AX7 and using the cheap and dirty 3m3/10 pf combo and then continuing on to the existing 3rd triode stage and CF before the tone stack. I guess I'd also have to use 2 triode sections after the reverb springs to get enough gain with an even number of sections to keep the phase correct when it mixes back in.
I've got some small xfmrs that are good for 3-4 watts, 2500 and 3000 ohms to 8 ohm sec.
I like the 6AQ5 because they're nice and small. I'll have to add an extra fil xfmr anyway but I'd like to keep the current down to have a 12vdc fan @ 300ma hung on the unregulated dc side of an LM317 (about 10vdc across the filter cap).
Any advice on which way to go or problems to avoid? Is there any reason to use one tube over another other than size or fil draw? Do I HAVE to buy/scrounge a higher value primary plate OT than 3000 ohms?
Please understand that I'm not trying to exactly clone a Fender 6G15, just do it in the same style and get a decent tone with only one reverb control pot on my crowded panel.
Also, I've got room for a full length Accutronics delay but will the medium length sound better to a 47 year old man who never wants to move on past SRV, BB King and George Thorogood?

1/20/2000 10:46 PM
Scott Swartz	Here is a reverb circuit I posted several months ago. You could sub in a voltage divider for the dwell pot described below. I should start by saying it is based upon on a circuit in Dan Torres book, Inside Guitar Amps, for an effects loop that splits the signal using a phase splitter biased by a tap on the cathode resistor (like a Princeton PI). It uses 2 1/2 tube positions as follows: first tube: 12AU7 1/2 tube is phase splitter 1/2 tube is gain stage for pan driver second tube: 6K6 or EL84, could be 12AT7 like Fender This tube is the pan driver 1/2 tube: 12AX7 This is the recovery amp from reverb pan The output of the amps tone controls couples to the grid of the phase splitter. The output of the phase splitter plate couples to the gain stage which couples to the pan driver. The signal from the recovery amp and the in phase signal from the cathode of the phase splitter are routed to a 1M mixer pot and the output of the mixer goes to the PI. The dry signal therefore sees only the resistance of the pot, or 500K when the pot is at its midpoint. A pot is placed before the pan driver tube, giving a "dwell" control. The circuit is placed between the EQ and the phase inverter, and I use a DPDT switch so the entire reverb circuit can be switched out of the signal path when reverb is not required. The complete circuit is approximately unity gain. By removing the large series resistance and the subsequent re-amplification, noise is much lower than the blackface circuit. This circuit gives very good control over the reverb tone and the dry signal is unchanged due to the massive headroom of the phase splitter (it can easily swing 30 Vrms). I think the circuit you described would have a gain loss if I'm understanding your post, since the dry signal will be attenuated through the 3.3M resistor and it would not be re-amplified? The two additional stages would amplify only the wet signal? Would it mix at the far end on the 3.3M resistor like a Fender BF?

1/20/2000 10:46 PM

Scott Swartz

Here is a reverb circuit I posted several months ago. You could sub in a voltage divider for the dwell pot described below.

I should start by saying it is based upon on a circuit in Dan Torres book, Inside Guitar Amps, for an effects loop that splits the signal using a phase splitter biased by a tap on the cathode resistor (like a Princeton PI). It uses 2 1/2 tube positions as follows:

first tube: 12AU7
1/2 tube is phase splitter
1/2 tube is gain stage for pan driver

second tube: 6K6 or EL84, could be 12AT7 like Fender
This tube is the pan driver

1/2 tube: 12AX7
This is the recovery amp from reverb pan

The output of the amps tone controls couples to the grid of the phase splitter.

The output of the phase splitter plate couples to the gain stage which couples to the pan driver.

The signal from the recovery amp and the in phase signal from the cathode of the phase splitter are routed to a 1M mixer pot and the output of the mixer goes to the PI.

The dry signal therefore sees only the resistance of the pot, or 500K when the pot is at its midpoint.

A pot is placed before the pan driver tube, giving a "dwell" control.

The circuit is placed between the EQ and the phase inverter, and I use a DPDT switch so the entire reverb circuit can be switched out of the signal path when reverb is not required. The complete circuit is approximately unity gain.

By removing the large series resistance and the subsequent re-amplification, noise is much lower than the blackface circuit. This circuit gives very good control over the reverb tone and the dry signal is unchanged due to the massive headroom of the phase splitter (it can easily swing 30 Vrms).

I think the circuit you described would have a gain loss if I'm understanding your post, since the dry signal will be attenuated through the 3.3M resistor and it would not be re-amplified? The two additional stages would amplify only the wet signal? Would it mix at the far end on the 3.3M resistor like a Fender BF?

1/21/2000 3:24 AM
Wild Bill	Thaks for the reply, Scott! I like your idea and will cheerfully steal it for the next amp I'm building. It does sound like a much better way to control "dry" and "wet". However, with this existing amp I just don't have the room. I've got just a small area in one corner to work in and I'll have to shoe horn in an extra fil xfmr, 3 sockets and the tank OT. There is only enough room on the front panel for one more pot, which is why I'm considering the typical Fender solution. I don't want to buy anything I don't have to - I'd like to work with the parts I've listed if I can. I might have to buy a 6MB8 triode/pentode just to cut out one socket and I'd really like to stay away from a large octal but I could if I absolutely have to. As I said, I just want something reasonably decent within these given parameters. If the consensus is that it would sound like crap (country?) then I won't even bother and I'll build something like the 6G15 "stand-alone".

1/21/2000 3:24 AM

Wild Bill

Thaks for the reply, Scott! I like your idea and will cheerfully steal it for the next amp I'm building. It does sound like a much better way to control "dry" and "wet".
However, with this existing amp I just don't have the room. I've got just a small area in one corner to work in and I'll have to shoe horn in an extra fil xfmr, 3 sockets and the tank OT. There is only enough room on the front panel for one more pot, which is why I'm considering the typical Fender solution. I don't want to buy anything I don't have to - I'd like to work with the parts I've listed if I can. I might have to buy a 6MB8 triode/pentode just to cut out one socket and I'd really like to stay away from a large octal but I could if I absolutely have to.
As I said, I just want something reasonably decent within these given parameters. If the consensus is that it would sound like crap (country?) then I won't even bother and I'll build something like the 6G15 "stand-alone".

1/21/2000 4:26 PM
Scott Swartz	With respect to the question of using the parts you have: 1. The 6AQ5 will work fine in place of the 6K6 or a paralled 12AT7, but I would suggest plate voltage 350 max. 2. The 3000 ohm trans should work. I implemented the circuit described above with a Hammond 125E trans (the one with all the taps) and tried various primary impedances from around 3K to 10K and didn't hear much difference. I settled on 10K since that is technically correct, but 3K should work for you. 3. Tapping off after the first 12AX7 to the 6AQ5 through the springs to two 12AU7 gain stages in series to a mixer should work. I don't think you would need the 3.3M resistor to tap the signal as you described. You could just have two coupling capacitors on the stage - one going to the tone stack 12AX7 and one going to the 6AQ5. The 3.3M is really just a fixed part of a mixer "pot". The inputs to your mixer would connect to the output of the tone controls and the cascaded 12AU7s. This is where the 3.3 meg resistor (and a pot for the reverb signal) would go if you use that approach. A dual potentiometer mixer as I used is probably more elegant, but the 3.3M will work. I will email a gif of my circuit and you can see what I'm talking about on the mixer. I have the drawing in AutoCAD R14 but I have to do some conversions to get it in gif. Give me a few days on this. My circuit uses 2 1/2 sockets, so it takes more space, but if you have three available it might fit. Another option is the blackface reverb circuit exactly as Fender used it, with a 6AQ5 driver the only change. This would require using only one 12AX7 and the 6AQ5, so would only need two sockets, one 7 pin and one 9 pin. One thing you will notice if you use the blackface circuit is that after the dry is attenuated by the 3.3M resistor and then reamplified by the 12AX7 there is a signifigant gain. Since the Bassman preamp has less tone control loss than the BF preamp due the the CF, your total preamp gain may be too much. You could use negative feed back to reduce it.

1/21/2000 4:26 PM

Scott Swartz

With respect to the question of using the parts you have:

1. The 6AQ5 will work fine in place of the 6K6 or a paralled 12AT7, but I would suggest plate voltage 350 max.

2. The 3000 ohm trans should work. I implemented the circuit described above with a Hammond 125E trans (the one with all the taps) and tried various primary impedances from around 3K to 10K and didn't hear much difference. I settled on 10K since that is technically correct, but 3K should work for you.

3. Tapping off after the first 12AX7 to the 6AQ5 through the springs to two 12AU7 gain stages in series to a mixer should work. I don't think you would need the 3.3M resistor to tap the signal as you described. You could just have two coupling capacitors on the stage - one going to the tone stack 12AX7 and one going to the 6AQ5. The 3.3M is really just a fixed part of a mixer "pot". The inputs to your mixer would connect to the output of the tone controls and the cascaded 12AU7s. This is where the 3.3 meg resistor (and a pot for the reverb signal) would go if you use that approach. A dual potentiometer mixer as I used is probably more elegant, but the 3.3M will work.

I will email a gif of my circuit and you can see what I'm talking about on the mixer. I have the drawing in AutoCAD R14 but I have to do some conversions to get it in gif. Give me a few days on this.

My circuit uses 2 1/2 sockets, so it takes more space, but if you have three available it might fit.

Another option is the blackface reverb circuit exactly as Fender used it, with a 6AQ5 driver the only change. This would require using only one 12AX7 and the 6AQ5, so would only need two sockets, one 7 pin and one 9 pin. One thing you will notice if you use the blackface circuit is that after the dry is attenuated by the 3.3M resistor and then reamplified by the 12AX7 there is a signifigant gain. Since the Bassman preamp has less tone control loss than the BF preamp due the the CF, your total preamp gain may be too much. You could use negative feed back to reduce it.

1/21/2000 5:38 AM
John Fisher	Scott I just happen to be doing the exact same thing as Wild Bill is doing. I am on the home stretch of building my amp which has 2 switchable Marshall preamp going into a power amp with 2 6L6's. I am trying to put in a reverb. Could you possibly send me that circuit that you were talking about? or where can I find it. I too have made this amp out of junkbox parts. I tried the Fender Deluxe Reverb circuit for my reverb and it has worked pretty good, but going thru the 3M3 resistor has lowered the overall volume quite a bit so I'm interested in seeing the circuit that you were talking about. Thanks a bunch!! John Fisher

1/21/2000 5:38 AM

John Fisher

Scott

I just happen to be doing the exact same thing as Wild Bill is doing. I am on the home stretch of building my amp which has 2 switchable Marshall preamp going into a power amp with 2 6L6's. I am trying to put in a reverb. Could you possibly send me that circuit that you were talking about? or where can I find it. I too have made this amp out of junkbox parts.
I tried the Fender Deluxe Reverb circuit for my reverb and it has worked pretty good, but going thru the 3M3 resistor has lowered the overall volume quite a bit so I'm interested in seeing the circuit that you were talking about.
Thanks a bunch!!

John Fisher

1/21/2000 4:27 PM
Scott Swartz	I will email a gif of my circuit and you can see what I'm talking about on the mixer. I have the drawing in AutoCAD R14 but I have to do some conversions to get it in gif. Give me a few days on this.

1/21/2000 5:50 PM
Wild Bill	Thanks, Scott! This is exactly the kinda thinking I was hoping for. I owe you at least two beer - Canadian, of course! -Bill

Page 1 of 2

Next>

Last Page>>