Modify Bell 3030 into a stereo guitar amp project

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I use a podx3 live and have had a hard time getting the sound that I want. Plugging straight into the mixer leaves it sounding... Well, digital... Thin with little feel and no liveliness to the sound. And, plugging into amp just doesn't sound right to me.(I've tried every combination of settings no no avail.) So, I've been looking for a stereo tube amp to warm up the sound of my digital preamp.

I've settled on a Bell 3030 Stereophonic amplifier from the late fifties. I love the warmth and can tell that I'm headed in the right direction. I don't need to do a complete conversion over to a guitar amp because that was part of the problem to begin with. What I need are some little tips and suggestions to try and get the most out of this amp. Like try this cap here or this resistor there, or I would try to get this voltage here or there by doing this or that... with a quick explanation as to what the adjustment will accomplish.

I plan to gut the "loudness" circuit, the bass/treble circuit and the riaa mess in the first preamp stage. I'm not sure what to replace it all with? I was thinking maybe the baxandall tone or something that will give me a flatter frequency response than the standard TMV tone stack to start with, but still give me a bit of tone adjusting after that.

Anyway, I've never posted before and am not sure as to what type of response I will get, but looking forward to hear all or any of your thoughts!

It's a 6v6gt stereo push-pull amp with two 12ax7 gain stages as well as a 12ax7 Phase inverter per two channels. ATTATCHED is the schematic.
 

Attachments

  • image.jpg
    image.jpg
    750.6 KB · Views: 434
Consider the 6SL7 in place of the 12AX7, -or- make a separate path and use that handy rotary switch. Try also different cathode grounding schemes, with different value resistors paralleled with any where from 2 mf up to 20mf. Also push-pull preamp going to the push-pull 6V6 finals. You have a bunch of real estate to play with. Do you have a resistor decade box to hook up in circuit & find what sounds good to you while you play? This is a huge time saver.

mike567 hit the nail on the head. Me, I say get ye a power & output tranny, some 5Y3s and have at it. Really, try the 8 pin 6SL7, extremely fat sounding, not tinny like the 9 pin stuff. The rear end looks good. you'll need a 330 ohm down to a hot 220/ 5 watt cathode bias resistor.
 
Unfortunately I don't have a resistor decade box...but I don't mind soldering. Time isn't an issue.

I'm assuming the big 5w would be for the 6v6. Am I correct with that? Can I run 12ax7 at the same time as the 6sl7's? If so would I just need two separate bias lines? Idk if that can even be done. I'm in the middle of self learning so I'm not the greatest with anything anything outside of the "normal" thinking. I'm not done learning though:)
 
I'm never going to be done learning...Well said

The 12AX7 is an exact copy of the 6SL7 (specification wise) except the latter is the bigger 8 pin version, and so is the sound also, not to mention the filament voltage is the actual only difference aside from the different pinout. This is a huge topic for another time & thread, but do some reading and you will see in writing the "how & why" everyone left this tube behind and went forward with the electronic miniaturization, and thus the thinner sounding 12AX7. Your Q: can they be operated in parallel? Perhaps, but I was commenting on the possibility of simply switching the inputs and sending the unused input directly to ground, possibly the HV DC supply as well. The rotary switch has become one of my favorite options in the one off amps that I champion, well it becomes a war actually. Love-hate war. But who/what ever wins, it sounds exceptional.

This gets me out of most of my tight spots......500pcs 1W Metal Film Resistor Assortment Kit 50 Value M | eBay

Sorry for the long link. Yes the 5 watter is for the cathode ground. You only need one. Just solder one end to ground & the other to a pair or just one cathode. You'll learn about checking tube bias with a 1 ohm resistor in series with the cathode grounding resistor and measuring millivolts which converts to milliamps of the tube being measured. Any data sheet on the web should tell you that the "Class A" Max watts (plate dissipation) should be. Then do the very basic math & you now know how hard your pushing the tube. (Ma x Volts DC (plate) will give you the watts your drawing) Good to have a ballpark beforehand, however w/ guitar amps, almost anything goes. Well you should at least try & find yourself at least once a few hundred percent above the Max ratings :) and enjoy the sound while you can. Just don't ever get any transformer above warm what ever you do! There way too expensive.

Consider striping the whole thing out. The time it will take you to trace everything. plus the head scratching, I'd start from scratch, since all the scratching will be from me, I can handle that. That rotary switch can be fun, fun. So many options in sound, compression, distortion. All from just adding different caps & resistors. A 5uF & a 4 meg in parallel on the 2nd pre amp cathode ( and others like 20uf & a 270K) to ground, 0.05 coupling caps, running the 112Vac power XMF at the higher line voltage: 125Vac and only knocking back the filament voltages (5.0V rect & the 6.3V with the dropping resistor).

Read, read, read. On these forums. Take your time & let us know what your Qs are sometime soon.
-Doug
 
........but before you (we) throw out the baby with the bath water, you may want to study what you have right there now. I don't know know your current level of tube circuit know how, but learning what you have as compared to the schematic would be a good starting point. Look for the coupling caps going to the grids, the cathode grounding through that big wattage resistor, the HV DC routing, outputs, etc.
 
Thanks Doug! Your response is appriatiated. It's not too technical and not too simple. I can understand enough to do some research.

........but before you (we) throw out the baby with the bath water, you may want to study what you have right there now.

It's funny you say that. I've been staring at the circuits for hours. Just taking it in. Just observing and matching the schematic to real life. I think I will take your advice. I'm waiting for my e-caps and after they are replaced I'm going to master this circuit. I will try things here and there before I do a complete gut job. However, I will have many questions. Should I limit my questions so people don't get annoyed or can I just ask away?

Question 1.) if all the coupling caps test good, should you keep them or replace them?

Thanks again for your input!
Paul
 
Have you tried the Pod into the Aux or Tuner inputs? They bypass the 1st stage.
Am I right presuming the pod has a stereo output?

Yes and yes.
The pod is stereo.

I have tried all set ups. For use with my pod the sound is very close. The lows are pretty tight sounding, but they need to be tamed down and it needs a little boost around 1.5k.

For use with just guitar It needs the bottom chopped off around 60-80hz the mids cut around 7-900hz and it needs a bit more sparkle.
 
Hi,

I'd leave it mostly as is. If anything rebiuld the phono /ceramic /
taped head section along the lines of guitar ampllifier input,
noting converting them to one input, the phono /ceramic /
tapehead switch positions could offer 3 different variants
of the basic input circuit.

All you need to do is play with the 3 different
feedback networks, and link the 3 inputs together.

rgds, sreten.
 
Last edited:
I agree with sreten.
Love the stereo option - some venues (eg Churches) need a big sound, not a loud sound. Your setup will do this. You have lots of opportunity for experimenting.

PS. I used a zoom 404 in just one loop of my stereo Church rig in the '90s. Mixed wet and dry signals to taste, and also had the wet and dry cabinets facing different ways.
 
Last edited:
Yes. I'm pretty sure I am going to keep it stereo. I am getting all kinds of ideas from your interaction and I haven't even received my e-caps yet:). As was suggested earlier, I am going to start small. Learn as is and then make small changes to see what happens.

I think the first thing I will do is experiment with djgibson's suggestions of playing with c19 and r23.

Then I think I might eventually go with MaicoDoug's 6sl7 idea. I could make two stereo sets of inputs. One using the tape/aux input and bypas the first stage. Then I can use sreten's idea and flavor the first stage for guitar without the pod amp simulator... Better yet! I can keep a stereo input at the second stage level and creat a mono high gain channel with the two first stages of 12ax7s.

There's an amp designed by 4 or 5 bloggers in just two days:) we will see how it goes?
I will still welcome more input but I think I've got a great place to start!
Thanks for all the ideas!!!
 
C19 I love to try a .05uf or smaller & C3 being 500uf? Wow. Largest I've ever gone here is a 47uf. Both of these mods will cut the deep bass.

Tell us what the stated input voltage says on the rear panel. If it says 115Vac, check the 6.3Vac filament supply under normal (no signal required) operation. The issue here is that the line voltages today are about 10Vac higher, which does place more voltage on everything. The filaments don't like much above 6.6 volts, this is with all tubes lit.

Your question regarding the coupling caps and should consider replacing them, well that's a good question. Dumble starts with a .002uf in the 1st stage of his amps. All depends how you like the current responsive curve & perhaps wanting to change it, also....how much have the current ones drifted, dried out, aged? The only way to really to know is to do some testing. Mouser has the Sprague "gum drops" for $1.60 ea. 0.01, 0.05, 0.02...I would lift one side up & check it w/ my handy cap meter to see how much it's drifted if at all. The input all of my cohorts have posted should give you a very good direction to start with.

The 6SL7 idea is for high gain inputs, like phono. Guitar is also a good use. Switching the DC bias "off" with one tube and "on" to the other, would make an interesting swap that you can do with the input rotary switch possibly. But I'm looking at this as a 100% guitar amp. Rarely if ever would one use am amp for both. I used to use my pioneer amp for practice, everyone liked the "clean" sound of 20 watts!

Your posted picture is sweet heaven, nice. Dual PP 6V6s. Easy 40 watts total out.

Doug
 
Last edited:
I use a podx3 live and have had a hard time getting the sound that I want. Plugging straight into the mixer leaves it sounding... Well, digital... Thin with little feel and no liveliness to the sound. And, plugging into amp just doesn't sound right to me.

There's a limit to what can be achieved with effects, it's true, but if you can't get somewhere close to the sound you want with the kit you've got, either direct or thru an amp, then maybe you are looking for a technological fix for a human problem. The complaints you have should be easily addressed with the existing controls on a mixer or amp of comparatively little sophistication.

It is possible that what you need is a change of guitar, Fender to Gibson, or vice versa, or even different strings. You can't expect to sound like BB King playing slinkies with a .008 top E.

Bear in mind though that a very great deal of the sound you get is down to the way you finger, strike, damp and bend the strings.

Very often when people become fixated on equipment upgrades to get the sound they want, it's a way of avoiding acknowledging their own technical shortcomings. Of course I can't hear you play, I don't know what you sound like, only you can know whether it's something that you might need think about.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.