ODS Reverb revisited

[ElectronAvalanche]

Hi gang,

I still had some problems with my 75W ODS with Reverb. Finally was able to fix it:

I folllowed Bob-I schematic (Reverb part only) using two 12AT7.

I had some nasty oscillation going on when turning up the Reverb Return volume: this was due to a shared cathode resistor for the Reverb Return amp.

Now it is like this:

Reverb tank out -> 220k to ground - grid (22M/0.1uF LNFB), Anode resistor 22k, decoupling cap 0.033uF, cathode: 820/22uF

Coupling cap (0.033uF) to Reverb Volume (250k) -> 470k/330pF to grid second system

Anode resistor: 47k, cathode 820/22uF, decoupling 0.01uF

The rest is as per Bob-I Reverb schematic.

The LNFB-Loop helps to get rid of the springy Reverb sound, smooths out the Reverb IMHO.

I played around with the second system cathode cap: in my case 22uF was nice, 4u7 is nice as well.

The 330pF sets the Reverb signal brightness together with the cathode caps. I dialed it in for a bright but still warm sound.

I still need to tweak the OD channel:

I have a switchable HRM Tonestack (Relay switching). I had a 0.0156 uF cap after OD1 and a 0.005uF on OD2. I changed the OD1 cap to 0.01 but don´t like the sound.

The OD values are 220k/3k3/5uF, OD input: 220k, 100k trimmer, 68k grid stopper,

0.01uF coupling cap, 100k series resistor to 100k OD Pot

150k grid stop on OD2, coupling cap of OD2: 0.005uF

HRM: 47k slope, 471pF Treble cap, 0.022uF Bass and Mid cap, 250k Treble trimmer, 1M Bass trimmer, 25k Mid trimmer.

The NON-HRM is a bit dull to me. Maybe I need to get rid of some shielded cable again?

Btw: I swapped the tube systems when trying to fix the oscillation:

Before I had the 6,7,8 system as the OD input system and this sounded much better IMHO. But I really do not want to change that back, since it is so much work.

I fixed the hum:

I need to isolate the input jacks from the chassis and ground them on the first ground point of the CL section.

The Reverb is absolutely hum free: I grounded the Return cinch socket, after isolating it from the chassis.

Grounding is ever so important in a tube amp!

Cheers,

Electron!

[dogears]

Hi Dom,

Try lowering the series resistance feeding the OD trim from 220k to 180k. I like the slightly brighter and more articulate tone. It lowers the miller capacitance of the OD1 stage as well as raising the the bass knee.

If you use shielded, make sure it is low capacitance. This is super important.

Still using 330pf snubbers???

What is a 6,7,8 system???

[ElectronAvalanche]

Heya Scott,

thanks for chiming in.

Snubbers are 250pF in this amp, the 330pF was a cap paralleled with the 470k series resistor of the Reverb stage.

I need to draw up a schematic. Anybody know of a free schemo program for a Mac computer?

A 12AX7 has two systems:

Pins 1,2,3 are anode, grid, cathode of system no.1

Pins 6,7,8 are anode, grid and cathode of system no.2

Supposedly the second system (6,7,8) is more sensitive. I looked at the datasheet, and indeed the two systems have a different instrinsic capacity (system one has lower picofarads as system 2).

I will try lowering the 220k OD input resistor. I might play around with the OD coupling caps as well.

Cheers,

Dominik

[Bob-I]

I folllowed Bob-I schematic (Reverb part only) using two 12AT7.

There's your first mistake

I had some nasty oscillation going on when turning up the Reverb Return volume: this was due to a shared cathode resistor for the Reverb Return amp.

I'm tempted to change this to parallel triodes. I don't feel the additional gain is needed.

Now it is like this:

Reverb tank out -> 220k to ground - grid (22M/0.1uF LNFB), Anode resistor 22k, decoupling cap 0.033uF, cathode: 820/22uF

Coupling cap (0.033uF) to Reverb Volume (250k) -> 470k/330pF to grid second system

Anode resistor: 47k, cathode 820/22uF, decoupling 0.01uF

Ok.....

The rest is as per Bob-I Reverb schematic.

Which I stole from the Fender AA763 schematic.

The LNFB-Loop helps to get rid of the springy Reverb sound, smooths out the Reverb IMHO.

I was blaming that on the tank. I have an old short tank that sounds less springy, but the reverb is too short for my taste. The new accutronics 2 spring tank has a nice long delay, but a springy sound. I'll try the LNFB.

I played around with the second system cathode cap: in my case 22uF was nice, 4u7 is nice as well.

I settled on 10uF.

Maybe I need to get rid of some shielded cable again?

After moving my relays and shortning some lead runs, I was able to remove some shielded. The amp opened up nicely.

Cool report.

[ElectronAvalanche]

Hi Bob,

no no, I was very glad you posted your schematic and ckt.

BTW: I tried the paralleled recovery section and it does NOT work. With a 100k anode resistors and shared 820 cathode resistors this produced too little output.

If you look at Garys Reverb-A-Loop, he has the paralleled section BUT another gain stage after that from the Dumbleator!

Last night it struck me that you use 22k and 47k as anode resistors.

I will have to check on voltages on the 12AT7. I really want to know how much mA are going through it. Maybe I will up the 22k a bit.

Cheers,

Electron

[Bob-I]

Hi Bob,

no no, I was very glad you posted your schematic and ckt.

BTW: I tried the paralleled recovery section and it does NOT work. With a 100k anode resistors and shared 820 cathode resistors this produced too little output.

If you look at Garys Reverb-A-Loop, he has the paralleled section BUT another gain stage after that from the Dumbleator!

Last night it struck me that you use 22k and 47k as anode resistors.

I will have to check on voltages on the 12AT7. I really want to know how much mA are going through it. Maybe I will up the 22k a bit.

Cheers,

Electron

I used the lower value anodes to reduce the swing. The gain was over the top, so much that the clean channels wouldn't clean up so I made 2 changes.

1) I returned the dry signal directly into the PI instead of the second gain recovery
2) I reduced the anodes loads.

[ElectronAvalanche]

Just as a follow-up:

I measured the voltages on the 12AT7 recovery tube.

Anode resistor: 22k for first triode after Reverb tank
cathode resistor: 820, 22uF cap

0.033 decoupling cap into 250k pot into 470k resistor with paralleled 330pf to grid of second triode with 220k to ground.

Second triode system:

Anode: 47k
cathode: 820,, 22uF cap

I have 3,8mA through the first system and 3,4 mA through the second. Which is quite a lot.....

Some more problems:

The amp sounds great, but has some hair on the Reverb signal. Some sort of overdrive on the Reverb which is still there with the Volume of the amp set to 0. Must be some ground problem still:

I put the grounds of the Reverb Recovery section to it´s filter cap ground, which helped eliminate a nasty oscillation. I will have to put the ground of the Reverb Send away from the OD and CLean cathode grounds.

I am still puzzling about the Reverb recovery section. Maybe I will go back to the 100k/1k5 combo for both sections again, but drop a lot of signal with a resistor to ground.

I am really undecided.

Wouldn´t it be for the hair on the Reverb and the signal bleed through, I would think the amp sounds quite nice. Maybe the Reverb sounds a bit too hot for my taste....

Cheers,

Dominik

[Tdale]

What schematic are you using here? The 3-tube circuit that Ben posted?

Tommy

[ElectronAvalanche]

Final tweaks and reworks:

I dropped the HRM/non HRM relay switching theme. I would have had to attenuate the Clean too much for equal volumes, since I have an overall Master and not the Clean Master and OD Master as in the HRM schemo.

I dropped the HRM altogether, since it took away from the 0.005uF into 150k sound. After I got rid of the HRM Post OD TS the OD really began to shine and got some highs back.

I swapped the 4k7 NFB resistor for a 3k9. A bit nicer me thinks.

I changed the Reverb AGAIN

I changed the 22k and 47k anode resistors on the Return to 100k each back. No distortion and the Reverb level range is ok with the 250k Pot. I lower the 220k to ground to 100k to loose some Reverb.

The 100k/820 ohms combo on the Return amp gives a better sound me thinks. A bit cleaner. Which is not at all what I expected.

I changed the lead dress when taking out the HRM, maybe this lead to the better Reverb sound with the new 100k/820 combo.

I think the 22M across the PAB really helps the Boost btw.

Cheers,

Dominik

[dobbhill]

Where exactly does this 22M in the PAB go, if you don't mind, sir?
Thanks, D

[Tdale]

What kind of reverb schem did you use? 3 tube? 1 tube..?

Tommy

[ElectronAvalanche]

Hi Dobhill,

with the PAB the connection between the Treble pot and the Bass pot is interrupted with one section of the DPDT relay and the Bass pots middle lug connection is lifted from the input coming from the 0.1uF Bass cap.

Just install a 22M across the floating contact and the corresponding NON-PAB contact on each side of the relay.

I hope I was able to explain it. A pic would probably say more than my text....

@Tommy:

I used the Reverb schematic as posted by BobI. Using two tubes.

CHeers,

Dominik

[dobbhill]

If I understand you, the 22M would go across the "open" contacts of the relay when the PAB was "engaged"?
I used a SPST relay and only disconnected the bottom of the treble control for my PAB, so I may need to re-address the disconnection of the bass control capacitor from the bass pot, yes?
Thanks much,
D

[ElectronAvalanche]

Hi Dobbhill,

indeed, the 22M go across the open and the "closed" contacts.

Cheers,

Dominik

[groovtubin]

Hi gang,

I still had some problems with my 75W ODS with Reverb. Finally was able to fix it:

I folllowed Bob-I schematic (Reverb part only) using two 12AT7.

I had some nasty oscillation going on when turning up the Reverb Return volume: this was due to a shared cathode resistor for the Reverb Return amp.

Now it is like this:

Reverb tank out -> 220k to ground - grid (22M/0.1uF LNFB), Anode resistor 22k, decoupling cap 0.033uF, cathode: 820/22uF

Coupling cap (0.033uF) to Reverb Volume (250k) -> 470k/330pF to grid second system

Anode resistor: 47k, cathode 820/22uF, decoupling 0.01uF

The rest is as per Bob-I Reverb schematic.

The LNFB-Loop helps to get rid of the springy Reverb sound, smooths out the Reverb IMHO.

I played around with the second system cathode cap: in my case 22uF was nice, 4u7 is nice as well.

The 330pF sets the Reverb signal brightness together with the cathode caps. I dialed it in for a bright but still warm sound.

I still need to tweak the OD channel:

I have a switchable HRM Tonestack (Relay switching). I had a 0.0156 uF cap after OD1 and a 0.005uF on OD2. I changed the OD1 cap to 0.01 but don´t like the sound.

The OD values are 220k/3k3/5uF, OD input: 220k, 100k trimmer, 68k grid stopper,

0.01uF coupling cap, 100k series resistor to 100k OD Pot

150k grid stop on OD2, coupling cap of OD2: 0.005uF

HRM: 47k slope, 471pF Treble cap, 0.022uF Bass and Mid cap, 250k Treble trimmer, 1M Bass trimmer, 25k Mid trimmer.

The NON-HRM is a bit dull to me. Maybe I need to get rid of some shielded cable again?

Btw: I swapped the tube systems when trying to fix the oscillation:

Before I had the 6,7,8 system as the OD input system and this sounded much better IMHO. But I really do not want to change that back, since it is so much work.

I fixed the hum:

I need to isolate the input jacks from the chassis and ground them on the first ground point of the CL section.

The Reverb is absolutely hum free: I grounded the Return cinch socket, after isolating it from the chassis.

Grounding is ever so important in a tube amp!

Cheers,

Electron!

FWIW on my amps, i only run (4) pieces of GEORGE L .155 shielded cable The input jack, clean master to second clean stage, OD preamp to second OD stage, and OD master out to the preamp out jack. LOTSA clarity no matter the circuit. Also a piece of mogammi from PA in to PI input so (5) pieces total... I like Gavvit stranded/soldered hook-up wire, use ta use it in the DIAZ amps I built back in 2004. Those amps were ALOT like a TR w/boost on and CRANKED to earbleed!!