What to do with unused triode ... suggestions?

[Diverted]

Well, today I received a giant pile of goodies from the Hoffman store and am ready to start next build. Each one is more fun than the last!
In the schematic below, I have removed one of the two channels I had in the original schematic/build, which people here helped me with tremendously a few months ago. I've removed it because the person who will be playing it won't need two and wants a simpler control panel. So as a result this will give me one unused triode on the 12AY7 preamp tube.

My question is: If you were building this amp, what would you use that unused triode for? Looking to have some fun and come up with something cool for it. Thanks! I'm open to anything; one thing I hope can be pulled off (but I don't know) is to find a single triode tremolo circuit that will work well in a cathode bias amp. Does one exist?

About the amp: It's basically a 5E3 with a Princeton reverb circuit stuck in the middle of it. When I first built it I didn't account for the decrease in gain that the reverb circuit would create, so unlike the standard 5E3 it has an extra gain stage.
There are a few things not on the schematic that I'm going to do (voltage doubler for reverb to bring down the level a bit, etc.) but nothing that will/should have to change due to the dropping of one channel.

Thanks!

[sluckey]

Here's a trem circuit that will work well with cathode biased 6V6s...

http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

[Diverted]

Thanks Sluckey. This looks great! A few questions:

1. Yours calls for a 6SL7, I'll be using half a 12AY7. Will I need to account for the differences in gain?
2. Also, the LED shown on your schematic. Is that the bug you see a lot on Fender amps? Have never built a tremolo with one of those bugs before so it's an unknown to me.
3. Finally, will lead dress be an issue? I'll probably just insert all the tremolo stuff where it makes sense, right next to the preamp area of the circuit board I'll build. Or should I move it?
4. As for the slow/fast switch, I'll probably just hard-wire one or the other (more likely the slow half of it) to cut down on clutter on the instrument panel.

Thanks for the great suggestion.

[sluckey]

Tremolo oscillators like a high gain triode. That 12AY7 may not work reliably. Just try it and see. You can always plug a 12AX7 in the socket.

The LED is just a simple red or yellow LED. Nothing fancy but you have to observe polarity. The LED may enable the 12AY7 to work. Try and see.

Lead dress is not an issue for the trem circuit.

[Diverted]

Not an issue on the 12AY as I have a feeling he's going to want the higher gain of the AX anyway. This is the perfect solution for that half a tube. Thanks again.

[Diverted]

How's this look for an LED? Correct?
So, I take it that as the intensity of the tremolo is turned up, the LED will grow brighter? And/or will it pulsate at different rates depending on speed? If so that would be very cool!

https://www.alliedelec.com/rs-pro-205577/70825080/?mkwid=snFxSRnoe&pcrid=30980760979&gclid=CjwKCAjw14rbBRB3EiwAKeoG_5vI_bOeiSVbOOe233GfnNzQfeRKQm-OmGIW3_DaN0Puy7Ay4zkoKRoCFU4QAvD_BwE

[sluckey]

That LED is fine. I have a similar LED bezel on the front panel of my Revibe and my Hampeg.

The LED will pulse at the speed of the tremolo. The brightness is constant, ie, it doesn't vary with the intensity pot. The LED will be off when the footswitch turns the tremolo off.

[Diverted]

Great. That sounds fantastic. Now to find one where the shipping cost isn't 20X the price of the LED 

[sluckey]

5mm red LEDs and bezels are dirt cheap on eBay. That's where mine come from. Also pretty cheap at Radio Shack if you still have a nearby store.

[Diverted]

Unfortunately all RS stores are gone locally.
I'm looking at this one, but there's no clue in the description about the ma rating. Link to data sheet does not work.

https://www.ebay.com/itm/1-Dialight-607-1112-110F-607-Series-2V-Red-Protruding-Panel-Mount-LED/201900452065?epid=1417968188&hash=item2f023468e1%3Ag%3AvXkAAOSw7U5Y-mVT%3Asc%3AUSPSFirstClass%2102809%21US%21-1&LH_PrefLoc=1&_nkw=2v+LED+panel&_sacat=0&_from=R40&rt=nc&LH_TitleDesc=0%7C0

[sluckey]

Something like this...

https://www.ebay.com/itm/150pcs-3mm-5mm-LED-Light-White-Red-Green-Yellow-Assorted-Emitting-Diode-DIY-Set/391368927437?epid=2284726737&hash=item5b1f67e8cd%3Ag%3AU5sAAOSwCypWpsoN&LH_PrefLoc=1&_sacat=0&_nkw=5mm+red+led&_from=R40&rt=nc

and this...

https://www.ebay.com/itm/20-x-5mm-Bezel-LED-Holder-Chrome-Metal-USA-Seller-Free-Shipping/232807780503?hash=item36346cd097%3Ag%3AaOQAAOSwBLlVUEMB&LH_PrefLoc=1&_nkw=5mm+led+bezel&_sacat=0&_from=R40&rt=nc&LH_TitleDesc=0%7C0

[Diverted]

You're right. Wow, that's crazy cheap. Thanks.

[Diverted]

Quick question about laying out the board:
Moving from right to left, V1, V2, V3, V4:
Would it make most sense to layout the board with components for:

V1A: Preamp
V1B: Second gain stage
V2: Reverb driver
V3A: Reverb recovery
V3B: Tremolo
V4: final gain/phase inverter

What do you think? I'm going to dry run through it without soldering, see how things flow.

[sluckey]

This is how the Princeton Reverb is laid out. I would do it like this also.
My reasoning... Reverb recovery stage is dealing with a very small signal, probably the most sensitive stage in that amp. Keep the wiring tight and feed it directly into the final gain stage in the same bottle. Tremolo is a BIGGG signal. So is the PI. Put them in the same bottle.

V1A: Preamp
V1B: Second gain stage
V2: Reverb driver
V3A: Reverb recovery
V3B: Final gain
V4A: Tremolo
V4B: PI

[Diverted]

Thanks Sluckey. That makes a lot of sense. Really looking forward to checking out the Ampeg tremolo.

[tubeswell]

... one thing I hope can be pulled off (but I don't know) is to find a single triode tremolo circuit that will work well in a cathode bias amp. Does one exist?...

The Fender Pawn Shop Excelsior has cathode-biased 6V6s and a singe-triode driven LFO that is buffered with an IRF820 MOSFET source follower

[VMS]

I was looking that excelsior schematic the other day, can someone enlighten me how that circuit works, please.

[shooter]

enlighten me how that circuit works

the same as most amps, stick a small signal in the left side, it comes out the right side bigger and modulated at the trem freq, IF trem is turned on.  I believe the PI is a cathodyne type.  the trem modulates the post PI signals.  It's self biased. guessing audio power ~~~~12-14w?

[Diverted]

Thanks for the other suggestions for the half tube. I think I'm going to go with the Ampeg tremolo. It looks very cool.
Anyway this is a question for Sluckey and anyone else who has an opinion. I started laying out the amp today using Sluckey's suggested layout:

V1A: Preamp
V1B: First gain stage
V2: Reverb driver
V3A: Reverb return
V3B: Second gain stage
V4A:Tremolo
V4B: PI
Here's the schematic:
https://imgur.com/a/xxOLgF2
I've gotten as far as the first two tubes. What do you all think of the routing/lead dress? Layout follows schematic except not shown is a mid pot at rear of the chassis, inserted between the tone/volume control and the input grid of the second gain stage; and a voltage divider not shown on the cathode of the reverb driver. Swapped it out from the original 1M resistor in that spot to reduce the reverb, which was pretty overpowering with the 1M.

Anyway, any suggestions or comments? Thanks!

[shooter]

nice start!
might wanna update the schematic for the next poor service guy looking for a 1meg R 

[Diverted]

Ha! Yeah I think I will. Just haven't made changes on the fly.
See anything in there layout-wise that's going to cause issues w/noise or other problems?

[shooter]

I've seen rats nest wiring quiet as a church mouse, perfect wiring sqealin like a pig.  take some of discarded coating from stripped wire and use it on your longer leg parts, justin case ya lean in to much

[Diverted]

Well I finished wiring the amp and fired it up this morning after going over the schematic a few times. All looked good, found no errors but I'm getting silence. Am going to have to go over it until I find the wiring error.
All voltages are in the ballpark At 110V B+ at 367, plates at 360, 330 ohm cathode resistor reading 20 watts. So the tubes are dissipating about 85 percent or so. Grids at 320.
Power up and down the rails is good and is reaching the plates. All plates, cathodes getting more or less correct voltage.
The tremolo is oscillating correctly as the LED is flashing and the rate changes with adjustment of the speed pot and the fast/slow switch.
Thought there might be an error with the circuit so I grounded it out at the footswitch jack. No dice. Same with reverb; grounded it out and no signal as a result either.

Sluckey, one question: The "X" point coming off the wiper of the tremolo intensity pot should connect to the junction of the 220K resistors in the phase inverter, correct? I removed the ground for those two resistors and connected them to the wiper.

I wish I could do more with the amp at the moment but I'm headed out of town for the rest of the weekend. I took tons of photos and will go over them to look for obvious stuff. Haven't seen anything that looks wrong yet, no points grounded that shouldn't be (checked with multimeter) ... But someting is up! I'll get to the bottom of it.

[tubeswell]

Do the pop test. (See Page 52 of Ch4 in Jack Darr book)

[Diverted]

Thanks for that link, looks like a great resource!
Before leaving I checked the all along the signal path with multimeter and also got Ac/DC voltages where I should be finding them. Probing produced no sound out of the speaker in any stage, and no trace of any sound with volume maxed. Did a bunch of probing for bad solder joints, all are tight. Checked output transformer, secondary and primary windings and they are fine. All tubes checked with a TV-10D/U before putting them in the amp and all are good. Eliminated speaker and speaker cable as well.
Am I correct in connecting the tremolo intensity pot wiper the junction of the 220ks feeding the output tubes?
Thanks!

[sluckey]

Am I correct in connecting the tremolo intensity pot wiper the junction of the 220ks feeding the output tubes?

yes

[PRR]

> no trace of any sound

Put your ear IN the speaker. _NO_ sound? Then the speaker is not connected or heavily shorted. (Or bad speaker....)

[Diverted]

Just got the problem fixed: A solder joint that looked good but was open on the final gain stage. So, back in business and I'm getting volume, reverb and tremolo. Sweet!

One question: The tremolo appears to be overdriving, or having some other issue. From about 1 to 5 on the volume dial, it works great (and sounds fantastic). When I turn volumei up past that, I lose volume completely and hear only the thumping sound of the tremolo. Any suggestions?

Thanks!

[Diverted]

CORRECTION:

It's not a tremolo issue. Grounded out tremolo and the issue w/volume cutting out about halfway up the dail persists, along with a regular thump (even with tremolo grounded). Is this some sort of motorboating issue?

[Diverted]

Here's a more or less updated schematic. The only things missing from it are a mid control at rear, pre-phase inverter master volume and a three-way impedance selector. Thanks for any insight on this motorboating issue!

Ted

[PRR]

> motorboating issue!

You have _four_ stages (lots of gain!) on _one_ B+ filter node.

Conservative design says no more than two stages on the same B+ filter. (This can be bent, but not in ignorance.)

Add a filter stage as shown.

[Diverted]

Thanks PRR. The suggestion is a great one. I did add another node and broke up the gain stage supplies. It helped to a degree but still got motorboating. I ended up dropping voltage to V1 from the supply down to about 115v. Low plate voltage, I know, but it's the highest I could get without motorboating, after subbing in lots of different resistors to test. At this point, with 115V on that first tube, the amp is responding well and smoothly. I thought it would be dirtier at such low voltage but it seems to be relatively clean ... in the neighborhood of what I was hoping for.
Specs on V1A and B:
Plates: 115vdc
Shared cathode resistor: 410 ohms
Cathode voltage: .5vdc

Thank you for your suggestion!

One issue I have to sort out is with the tremolo, which seems to be very powerful in that when I dime the intensity pot, I get a click with each pulse of the tremolo, when I apply signal from the guitar. I am going to experiment with some resistors, per a suggestion from Sluckey, to get that click to disappear since I am using the LED (very cool feature by the way)!


Thanks!

[PRR]

> highest I could get without motorboating

Bigger caps are as effective as bigger resistors, but no added voltage drop. What uFd are you using?

[Diverted]

Thanks for responding!

Filter caps are 40/20/20/20
Cathode bypass caps in preamp and first stage, 22uf.
coupling caps throughout .022.

Just got some voltages put together. Biased right around 98 percent. Here's the full numbers:
B+ 360
B+1 296
B+2 221
B+3 204
Output plates 353/354
Output K (250 ohms) 18

Phase inverter Plate 164, K 42
Last gain Plate 138, K 1.2
Tremolo Plate 188, K 1.8
Reverb return: Plate 134, K1.2
Reverb driver: Plate 297, K6
Gain 2: Plate 78, K1.1 (12AY7). With 12AX7 around 115.
Pre: Plate 74, K1.1 (12AY7). With 12AX7 around 112

[Diverted]

Quick follow-up question to try to keep this going. At this point the main thing I'm wondering about is the effect of those V1 voltages on the 12AY7 tube. Does running the amp at these numbers risk harming the tube or causing reduced life and/or other issues?
These voltages have eliminated the motorboating but I don't know what effect they will have long term.
Thanks for your insight!

[sluckey]

I ended up dropping voltage to V1 from the supply down to about 115v.

How did you do this?

[Diverted]

I soldered a 15k resistor to the junction of the two plate load resistors at V1 (82K and 100K). Then I attached the power supply to the other side of that resistor. That brought the voltage down to where it is. Any lower value on that resistor and I got motorboating, so that was the minimum number that seemed to do the trick.

That 115 is the plate voltage with a 12AX7. With an AY it's about 80 or so.

[sluckey]

I soldered a 15k resistor to the junction of the two plate load resistors at V1 (82K and 100K).

You also need a 22µF filter cap at the junction of those three resistors.

[Diverted]

Thanks Sluckey. Will that last cap serve to increase voltage to V1?

Coincidentally this is the fourth of this circuit that I have built but the first to have this motorboating issue, and the only difference between it and the others is the insertion of the Ampeg tremolo circuit and the removal of one channel to free up a triode for the tremolo. A few people had suggested to me that the tremolo was the root cause but it isn't; to make sure I completely removed the tremolo from the circuit by removing the jumper from the intensity pot wiper to the junction of the two 220K resistors feeding the power tubes, and running those straight to ground as in the original, and also removed the power feed to the tremolo triode's plate. Even with those two tie-in points to the circuit removed, I still got the motorboating issue so it isn't that.
Some had suggested that I had too much overall gain in the amp and that was causing the motorboating, but I wonder about that as previous versions of this amp have had no motorboating issues, with the same gain stage layout.

[shooter]

most of my builds use a AU7 in V1 running between 70 - 90vdc at plate, NO problem.
you're starting with a mV signal, swinging it up to a volt or 2, there's no reason you need 200+vdc to swing such a small signal.  You DO want the B+ stable, small AC on the DC will make a big headache

[Diverted]

Thanks shooter, that's good to know. So definitely then per Sluckey's instructions run a 22uf cap to ground from the three-resistor junction to keep the AC out, and I should be good to go on that triode? Thanks. How about the bias? Obviously V1 is cathode biased so its self-regulating (doesn't need to be fixed) but if I'm doing the math right it looks like I have super low dissipation on that tube. Is that an issue?

Not noted in schematic but I replaced the shared cathode resistor in V1 from the original 820 to 560.

[sluckey]

The cap puts that new B+ node at AC ground just like any other filter cap. Without the cap the signal at the plate of V1a and the signal at the plate of V1B are mixing together and will cause problems.

Not noted in schematic but I replaced the shared cathode resistor in V1 from the original 820 to 560.

     WHOA!!! Are you saying that V1A and V1B are sharing a cathode resistor? That's a big no-no and is likely the cause of the motorboating. You cannot allow two consecutive gain stages share the same cathode resistor. Give each triode it's own 1.5K and 22µF and I bet the motorboating will go away.

[Diverted]

Yes, they do share the same bypass cap/resistor.
Previous version DID not as I had one channel voiced differently than the other. I figured I would save space on the board this time by doing a shared cathode.
I'm learning this stuff as I go and did not know that was a no-no in successive gain stages. That is very good to know!
Now to sneak away from work while nobody is looking and see what I can do. Thanks!

[sluckey]

Let us know how it turns out. I still recommend using the extra B+ node as you have it now, but add the filter cap. And you can probably reduce that 15K to 4.7K if you want a little more voltage.

[sluckey]

PRR made this post a few years ago...

https://el34world.com/Forum/index.php?topic=11053.msg101896#msg101896

[Diverted]

Success!
I split the cathodes on the preamp and first gian stage, and took out that last 15K resistor (that I only put in because it was the lowest value that would stop the motor boating). So with cathodes split and pre and first gain plates running directly to original power supply node, the motor boating is gone. With the 12AY7 I have about 105-110V on the two plates.
Thank you both very much for the suggestion, and for the link to the previous thread about this. I didn't know a feedback loop was the result of sharing those cathodes. It's just nothing I had ever come across before, and had no idea was an issue.
Getting back to the power rail: I am sure I need to adjust it as I have four plates on that last node. How would you suggest breaking it up? Moving one of them to the node that feeds solely the PI?
And/or do you still think I need a fifth electrolytic tied in at the last spot?
Many, many thanks for helping get to the bottom of this. Makes my day.

EDIT: Sorry, in photo forgot to addd the values of the resistor. From left to right they are 5K, 18K and 5K. I may play around with those once I get the node situation figured out, to get the voltages back to what specs say.

[sluckey]

How would you suggest breaking it up?

You already had it. But you NEED the additional filter cap. And reduce the size of the 15K to 4.7K.

DON'T USE THE PI STAGE B+ NODE FOR THOSE PREAMP OR REVERB RECOVERY STAGES.

[sluckey]

Just for a bit of info... The very popular and successful Fender AB763 amps supply SIX stages with the same B+ node! I always thought that was a bad idea and would never design anything like that. But it served Leo quite well.

My point… It's a better idea to split up the B+ nodes, but it sure ain't necessary. I don't mind spending the extra few dollars to have a 'better' B+ rail. Now, you decide what you want to do. 

On another note... Have you reconnected the tremolo? Did you use the resistor suggestion? Thumping gone?

[Diverted]

Sounds good. I will add an additional 22uf at the junction of the plate load resistors feeding V1, and move the PI stage from node D to node C.
One thing I'm not following: You mentioned reducing the 15K resistor to 4.7K.
If you are referring to the one I previously added to the junction of the V1 plate resistors and from there running to the power supply, I already took it out when I split the cathodes. So, V1 plates now run directly to Point D on the drawing of my power supply.

As for the tremolo ticking yeah, it's pretty much gone. At very high volume and with the feedback switch set in Fender 5E3 mode (no feedback, 22uf  cap in circuit) I still get very slight, slight ticking when I have all the dials pegged. You'd suggested a 470K resistor to the intensity wiper to start. I tried that and moved up until I hit 1M. With that one in, it's still very, very slightly there but only with volume dimed.

Thanks for the suggestions!

[Diverted]

I don't mind spending the extra few dollars to have a 'better' B+ rail. Now, you decide what you want to do. 

I couldn't care less about what it costs. I want it done right, always! That's my only consideration. Thanks!