One Tube reverb - dwell control problem?

[Shamadan]

Hi. First of all thank You so much for all the information that You share on this forum it's really great. I've recently built tweedle dee with one tube reverb. Tweedle has some cools tons that I really dig. Yesterday I made one tube reverb circuit but I am not 100% sure if it works right.

With Dwell on 2 Reverb 10 I have some small reverb that I like but when I start to increase dwell past 3 sound is geting distorted. Is it normal? Can I somehow fix it. I would like to have more reverb that wouldn't distort sound.
(I replaced 1nF cap and 12ax7 to 12at7 but distortion was unaffected)

Schematic attached. Thanks for help.

[pdf64]

What Vdc on terminals 1,2,3 of V2 at idle, all controls at min?
With a 12AX7 and 12AT7.
Try muting the power amp (eg pull the second 12AX7) and connect a speaker in place of the output to the reverb tank. See how that sounds, using the EF86 channel.

[10thTx]

Not normal. Should not have distortion. I would suspect something is not wired right, wrong reverb tank, something wrong with reverb tank.

Normal for me with one tube reverb would be dwell on 6 and reverb on 3 using a 1M reverb pot.

Any chance you hooked the wires to the reverb tank backwards? What happens if you bang on the reverb tank?

http://el34world.com/Forum/index.php?topic=7957.0

with respect, 10thtx

[Shamadan]

Reverb Transformer classictone 0-18034
Tad Reverb 4AB3C1B

Dwell control sounds a bit like a gain stage - more increase Crunch sound (Sound is clean when dwell on 0 - no reverb)

V2 -second gain stage
6 180
7 ?
8 1.5v
Phase Inverter
1 228v
2 16v
3 44v
Reverb tube 12AT7
1 322v
2 0v
3 5,3v
6 133v
7 0v
8 2v

Swaping rca cables - no reverb
Transformer red wire to B+
blue to triode
grean to rca tip
black to ground
banging the tank - Can hear springs a bit - dwell doesn't effect sound

I did few more tests and I think it is not right dwell effecting sound from clean to crunch when Reverb controll set to 0.
Thanks for helping

[10thTx]

I don't see anything wrong with your voltages. If your layout matches your schematic, it should work fine. Try another reverb tank if possible. Maybe one of the wires inside the tank is bad? IF it is not the tank or tubes then I would suspect a simple wiring error?

With respect, 10thtx

[Shamadan]

Thanks 10thtx for confirmation. Unfortunatly I don't have an extra tank. I have 8eb2c1b in diffrent amp but Impedance don't match. I chacked multiple times for wiring error but will chack once more.

[pdf64]

Unfortunatly I don't have an extra tank

If it's an 8 or 10 ohm input transducer on the tank then rig up a speaker in place of the tank input.
Then you can assess the quality of the signal.
What taper is the dwell control?
The signal level there may be fairly high, and the 12AT7 will clip when it goes >~3.5Vrms.
So an audio taper may work better for the dwell than a linear type.

[Shamadan]

Thanks pdf64 for great idea will try it in 2 hours and see what hapends.
Dwell is an audio taper.

[sluckey]

I did few more tests and I think it is not right dwell effecting sound from clean to crunch when Reverb controll set to 0.

I understand this to mean it's not the reverb sound that becomes distorted. Rather, it's the dry signal that becomes distorted when you turn the dwell control. Is this correct?

If so, unplug the reverb tube. Does the dwell still make the dry signal sound distorted?

[Shamadan]

Yes sluckey You are correct
Without reverb tube dwell doesn't change sound

[sluckey]

Does this distortion appear on all channels? Verify the circuit connection. Try another 12AT7. Also try a 12AX7, 12AY7, 5751, 12AU7.

You may also try a 33K to 100K resistor between the dwell pot wiper and grid of the tube.

[Shamadan]

I tried few things
1.Connected speaker to reverb transformer - (funny that you can play like that)
Distortion is still present, but now i will redefine it to harsh sound under normal sound
2. change reverb tube to 12au7 still harsh sound
3. grid resistor to pin 2 of reverb - no change
4. Tried both Tweedle dee channels (EF86 circuit is not connected until I solve this issue)
Whan I use reverb tube - there is an extra harsh sound when dwell above 5 reverb 0-10. When reverb is above 5 I can hear more harsh sound
So I think that we narrow down the problem to reverb driver triode and its connection.

I will check everything tomorrow. Thanks for everything.

[10thTx]

I know you've probably checked numerous times ........... but please double, triple and quadruple check your wiring and the component values.

I am still thinking the tank is bad or the wiring is incorrect?

If you have the EF86 channel disconnected and with no tube in it, I don't see how the reverb can distort the signal like that unless:


- wiring is not wired properly
- component value is wrong or not correct reverb trannie
- wiring to tank is reversed accidently
- tank is not working properly (you can use your voltmeter to check
continuity and resistance)

(I've seen people fail to remove the packing inside the tank that keeps the springs from moving and also the wiring inside the tank come loose & be defective. Also seen where the springs were sagging & hitting something on the bottom of the cab )

There just isn't that much there to cause a problem. I have used the one tube reverb on quite a few amps and never had the problem that you are describing to the best of my memories. And the reverb has always worked for me.

With respect, 10thtx

[Shamadan]

I can't find any mistakes - but am sure there are some. I checked wiring few times and for now everything seems fine. I tried diffrent grounding schemes.
I can control those distortion quite well while playing. With dwell under 7 I have to hit strings quite hard to hear those unwanted sounds.
For now I will connect ef86 circuit - Enjoy great amp and try to figure it out when I get a chance.

I don't think its thank related problem - distortion happens without rca connected - no reverb tank connected - Driver triode somehow change tone - Probably some mistake under turret board but it will take ages to get underneath - will do it as last resort.

Thanks for help

[Shamadan]

Hi.
Still trying to solve it. I did check volteges in my amp. With reverb tube
all valeges after first 4k7 resistor drops 30V. So
B+ 380 Screens 330 B3 311 - without reverb tube
B+ 380 Screens 300 B3 280 - with reverb tube

Sorry to bother you guys but is it normal?