5E3 cascading inputs MAD FEEDBACK! Need fix

[Chodrick1]

I've got a Mojotone 5E3 that I've been fiddeling around with and today I did the "Cascading Inputs Mod" from Rob Robinettes website. I love the gain tone, I love how I can swap back between the standard 5E3 and this drive monster. Unfortunatley I have run into a bit of a problem which seems to limit what I can do with it. The issue is that independent of where the gain volume is, turning the master volume or tone to 9 or higher, I begin go get terrible microphonic feedback, and cuts the guitar signal out completely. I'm reletively new to amps, so I'm not sure what may be causing this. I would love to get this amp working 100%, but this could be a real pain in the butt if this is just how the mod works.

Ive got the standard tube layout, 470 Ohm 5w screen resistors on my power tubes and 300k grid resistor out of the phase inverter. Hopefully this will help narrow down any issues.

Thanks ahead!

[Chuck H]

Too much gain and possible lead dress issues (can't see that part). Looks like too much low end through the preamp for a cascade high gain circuit also, though that's not what's causing the oscillation. Possible microphonic preamp tube issue but I think the circuit would oscillate anyway. To effectively employ this much gain it's usually necessary to use VERY careful layout and bleed some top end out of the signal chain to keep the amp stable. I also don't like the way the inputs are rewired. It removes nearly all options normally offered by the four input arrangement, so what's the point in having four inputs after that? I don't like this mod. IMHO it's clunky and poorly considered. I promise you that Rob Robinette has never built this amp with stable results. I'm sorry I don't know what to say beyond this right now. The circuit might be redesigned to work, but since you're new to this I hesitate to try. Taking on a really high gain circuit is tricky enough for experienced builder/designers. It's A LOT more than just the schematic. It involves component layout, grounding, voicing and phase and noise considerations.

Sorry to be such a bummer. There's just so much to fix about that circuit and there's not much to the circuit. It would be better removed.

[Chodrick1]

It's alright Chuck, I appreciate the honesty.
I walked into the modification not knowing exactly what to expect. With the kit that I recieved, it came with a 35 watt speaker, which keeps almost all of the gain from moving the speaker, at first my amp would never breakup, it was entirely clean, which wasn't bad, but I chose this specific amp for the overdrive tone, and the problem became that I wouldnt be able to be buy a replacement speaker with a lower output rating for probably a couple of months. I wanted to find a way to drive the Deluxe harder, given my current arrangements. Putting the screen resistors on the 6V6s gave me breakup at 10, but that is very mild breakup even at max volume. I tried a 12AX7 in the V1 socket for more output, but this seemed to only make the amp brighter and messier.

Do you have any suggestions on how to achieve this?
Or any recommendations on how to improve the cascading inputs?
Or, would it help if I placed resistors in series with the volume and tone pots so that I cant achieve a wide open signal, this way I will max out right before I start to recieve feedback?
I'm just thinking out loud, but I would think that would help, however I am not sure what else this would effect.

[Chuck H]

Well I visited Rob's site. Nice and navigable. Good drawings and graphics. He does make mention of reducing low end through the circuit for the cascade mod. His lead dress looks good too, but the layout is still probably not up to the amount of gain. Though it appears he HAS built the circuit a couple of ways. With the jack switching and with a toggle for switching. I have to wonder, though, if he ever had a stable circuit with the component values posted in your link. IMHE it would be surprising.

A 5e3 should clip pretty well when cranked up. Not hard rock or anywhere near metal clipping levels, but still obviously distorted. If yours doesn't there may be another issue that should have been corrected before attempting the gain mod.

I had a 5e3 cascade mod post here once. Not sure where it is, but I'll look. Admittedly I never built it because I don't have a 5e3. But I'll try to find it, see how it looks and post it if I think it's appropriate.

Incidentally, are you using the pre PI or post PI master circuit?

[Pedro Vecino]

A at first my amp would never breakup, it was entirely clean, which wasn't bad, but I chose this specific amp for the overdrive tone, and the problem became that I wouldnt be able to be buy a replacement speaker with a lower output rating for probably a couple of months.

A Deluxe has a lot of distortion, but in its own way. It is one of the least effective concepts of what I know, and also one of the most delicate to obtain musical results. Is needed "old style technique" to play with it because it handles a lot of bass and a raw sound with a very "asymmetrical" overdrive: an explosive mixture. :-)
I would never mount that circuit on that amplifier.

[Chodrick1]

Im not using a master volume. I cleared a spot from the mic volume, and I'm using that pot for the drive control. Ive just got a Drive Control, Bright Volume, and Tone.

[olddawg]

Honestly... have you simply tried it with just a TS type pedal first? Bad Monkeys are cheap, less time consuming, and the amp is usable.

[Chuck H]

Chodrick1,

I found the old thread, but I don't think my own version is a lot better. It's very similar to the switch version of that mod, except that I added a cap in series to reduce bass, mine is a little lower gain and mine doesn't have the drive control. It was intended to be a very simple gain boost for another poster and I figured it would be refined after it was wired in. But the thread petered out. Overall not significantly better than the mod you already tried. I did start to draw up some refinements, but it started to become prohibitively complicated. Not for me, but for any unsuspecting modder I might inflict it on It involves one of those dual iso switch jacks with nine lugs. It would be silly tedious, but if done it would make it so the amp would work as stock, look stock and stack the channels when plugged into just the one jack. No external switches. It's also voiced more appropriate for the higher gain and stability. Lead dress and layout would be critical. Since I'm already halfway through the drawing I'll go ahead and finish it if your interested.

[Chodrick1]

Absolutely, if you wouldnt mind. Im open to new ideas!

Do you think I could eliminate the feedback by lowering the amount of gain, possibly by altering the cathode bypass cap and resistor. I agree that the gain on it now is outrageous, I want mild to cruncy distortion, Im not playing metal or anything like that, I want punch and a bit of natural overdrive. Which I have both of those with this mod. Like I said, I absolutely love the results except for the issue with feedback.

I do have a Ibanez Tube Screamer, however I want an all in one type of amp. I know that is quite a lot to ask for, but the things that I was looking for in the amp are now present, but it does come with the issue of feedback.

[Tom Phillips]

...With the kit that I recieved, it came with a 35 watt speaker, which keeps almost all of the gain from moving the speaker, at first my amp would never breakup, it was entirely clean, which wasn't bad, but I chose this specific amp for the overdrive tone, and the problem became that I wouldnt be able to be buy a replacement speaker with a lower output rating for probably a couple of months...

Jimmy,
Although the speaker makes a significant contribution to the sound of an amp the wattage rating of a speaker does not affect the performance in the way you stated. If you elaborate about what you mean and/or where you got this idea then maybe we can better understand what you are getting at. A low wattage amp can work just fine with a high wattage speaker provided that it is a good speaker with normal efficiency. In addition, high gain amps do not depend on distortion caused by over-driving the speaker. For now, I recommend that you focus on getting the electronics sorted out. Then you can experiment with different speakers if necessary. Just understand that the "...keeps almost all of the gain from moving the speaker..." idea isn't a valid concept and you do not need to worry about changing speakers at this time.
Cheers,
Tom

[Chuck H]

Well here ya' go. It looks more complicated than it is because I drew it accurate to the switch action and that made for more crossing leads in the diagram. The low gain jack for the mic channel would become the channel stacked jack. The volume for the mic channel becomes the "gain" knob when using that jack. Because it's not wired like a standard voltage divider the taper may be a little funny for adjustment, but it should work just fine. Considering the refinements over my older idea and the Robinette circuit above, it's really not that complicated. Refinements include:

Gain of the circuit attenuated a bit for better control and more intuitive adjustment.

Revoiced with appropriate LF/HF roll off for higher gain voicing and stability

No added knobs or switches. Amp looks entirely stock.

The amp and the other three inputs work exactly as stock when not using the high gain jack. So you can still do things like jumper the stock channels and use them parallel or A/B switch between them.

*NOTE: The mod will probably work fine with the shared cathode circuit on V1 BUT YOU CAN'T REDUCE THE BYPASS CAP VALUE. If you do wish to reduce the cathode bypass cap value you will have to separate the cathode circuits.

Here's the diagram and a link for the jack you'll need to do it.

9-lug Stereo Jack | stewmac.com