Mod'ing a tube circuit (from early 60's stereo turntable)

[TubeNoob]

My main intention for this unit was for re-amping stuff that needed some pizzazz and was super glad to crank it up this weekend and do just that. I had a guitar line that even though it was tracked through an amazing 60's Bassman with killer mics and preamps in a great studio, it was lacking in upper frequencies. (crazy, i know) This box did just the trick! Was really cool to switch things in and out of the circuit and use that mid-boost pot to dial in the right sound. Yahoo!

[TubeNoob]

A strange thing happened on the way to jamming out... One channel stopped working. DRATS!

When I remove the tubes and test voltages I am getting a report from the speaker when I check the plate on the EL95.

The issues I have found:

1) Was very hard to get a reading for the 2nd plate (pin 6) of V1b with my probe, which fits down into the pin hole.
2) Pin 6 showed 280v when I finally got a reading. Pin 6 of the other channel shows 265v.
3) C12 is not showing any voltage at all.

I'm going to:

A) Clean the pin hole
B) Check solder connections for Pin 6 and C12

Anything else I should be looking for?

[Chuck H]

You may have a leaky cap putting voltage on the chassis. That's all I can think of to cause a report from the speaker with no tubes in. Did you install a chassis safety ground?

The difficulty getting a reading from pin 6 of the preamp tube could be a short, a fault or just old grimy flux preventing good contact. One tip I read here was to sharpen the meter probes for poking through that stuff. I haven't done it yet but it seems smart. Check voltage on the other side of the 220k resistor connected to pin 6 If it's still troublesome to read you may have a problem with the voltage rail.

[TubeNoob]

Hi Chuck!

Regarding the speaker report, it's when I test the plate voltage of the output tube. It does the same on the working channel, as well.

I did check that 220k (R19) and it's putting out 280v. That's what's strange to me, the cap (C12) next to it is not seeing any voltage.

Yes, i did put a grounded cable on it and tied it to the chassis.

[TubeNoob]

Ugh, been out of the loop for a while due to my first cold in probably 5 yrs... But i'm back!

So, i did check R19 as i said, and its getting 280v. C12 is not seeing any voltage.
I checked continuity from the tube socket to R19 and C12 and its just fine.

Really strange to me and I admit I'm not sure what to check next on this thing!

[g1]

So there is 280V on one side of R19 and no voltage on the other side?
Does it check out at 220K? Are both heaters lit at V1 ?
Does V1 pin 8 measure about 3.4K to ground?

[TubeNoob]

Hi g1, thanks a lot for those ideas; I checked those and here's what I found:

V1b on Good Channel - Pin 8 to ground: 3.5k
V1b on Bad Channel - Pin 8 to ground: 3.5k

R19 on GC: 200k
R19 on BC: 220k


So, then I re-flowed the solder over the pins and R19 and the dang thing is workin!!! Yahoo!!!! I guess something got jiggered loose during my mod work; though i did find a little component lead floating around beneath the circuit board which i removed, so it might have been that?

Anyway, if my luck holds then there shouldn't be anything left to do on the PCB and i can go ahead and fix it in place.

Just tried both channels with this knock-off Strat i just got for 30 clams, (my usual Strat-knockoff is a Samick that is really dark sounding, so it was nice to hear the amp with that crazy Strat chime). This amp kills it! Very different sound from my Princeton build, that's for sure. Apples and Oranges, though.

Thanks again for everyone's time and expertise!!!!!

Still working on the effects/mixer stuff that will go in place of the turntable. Having issues with the dual PS where it checks out fine (+15 / -15) when not under load, but the negative side is collapsing down to 2v under a test load. Tube stuff is so much easier to work on ;-)

[TubeNoob]

Time to WAKE this thread back up! Hi guys!

This project is getting pretty close to completion!

So I have been slowly tinkering with effects and also treating my steel sheet to the elements to give it some visual vibe for the front panel.

So far I have built the following things to go in the box:

1) Germanium Fuzz Face (2n527 transistors)
2) Dallas Rangemaster (2n527)
3) Two AMZ tone controls (the last one on his page, with boost and cut)
4) Cave Dweller Delay (still working on this one)
5) Almost forgot! I designed my own Muffer distortion that sounds really dang cool.

I still have a few simple things left to build including the FX sends, 6 channel mixer, buffer units, etc.


Something I really want to add is tremolo. But instead of doing a stand-alone solidstate effect, I wanted to ask y'alls opinion on how to do this:


Solidstate LFO driving the cathode of V1B

(Check out the Kalamazoo Model II: http://www.rru.com/~meo/Guitar/Amps/...ooM2Schem.jpeg).


I had one of these amps for a while and the Vibrato seemed to change the distortion more than the volume.

Any ideas how to work this one out?!

Hope everyone's been doing good!

[TubeNoob]

Long overdue for an update on things...

First up, here's a quick recording I did yesterday trying the amp out. No FX at all, just two mics straight into the soundcard; only balanced the L/R volume a bit.
The first few riffs are SM7 and SM58 on the grill; last riff was two SDC mics a few inches from the grill.

https://soundcloud.com/user-34254870...ono-amp-test-1

Couldn't be happier with the sound, very punchy, nice clean sounds, breaks up pretty well. I didn't crank it all the way as I need to replace some long wires with some short shielded cable or it will start ice-picking your ear.

In the photos you can see the new faceplate. After leaving it out in the elements all winter then giving it an acid bath it turned out nice and beaten looking. Then drilled it out and got a local metal shop to bend the sides for me. Going to color the metal with heat.

I got the delay working, and I went ahead and built an external tremolo. It's a little more square wave than sine wave but its cool enough.

I want to coat the faceplate with polyurethane, but still have to figure out the right solution for labeling all the controls.
Would love to hear suggestions!!

And now some photos:

The new faceplate - here i've installed all the controls for the amp stuff. Those controls are (for both left and right channels):
Negative feedback; Lift tone-stack; mid-cut; mid-boost with potentiometer control, cathode bypass cap, 100pf bright cap, volume controls.
All the other holes are for effects, the mixer and some multipliers. 85% done on all those other circuits.



The close-mic arrangement with SM7 and SM58 -



The effect circuits that will be going inside -

[Chuck H]

That's a great, and useful tone. More of a balanced clipping on LF and HF like good tones for blues, country and some rock and hard rock. I think I can tell it's a pretty nice sounding guitar you have plugged into it too. My own "tone" is definitely more clipping in the upper mids for definition under heavier clipping. But there's been many times I considered making adjustments so my own amp could do what yours is doing

Nice.

[TubeNoob]

Hiya Chuck!
Thanks a lot for the awesome feedback.
I am stoked to hear you say that, and stoked with this amp. The first three riffs on that recording were all the same mic'ing and settings - the differences were only due to how hard i hit the strings; so it sure does seem pretty versatile. Once i get those shielded cables installed i should be able to crank up the mid-range. I do also still have the bass/treble controls from the original amp in the circuit (optional!) so i can try to go for that mid-range tone better in the future.
And I have you to thank - all the changes to the circuit were your suggestions, so your own amp is in very good hands should you decide to mod it! ;-) But i'm here for you if you need any complex/improbable solutions for any simple problems you might have... heh hehhhhh.
Cheers and thanks again!!!!
-Steve

PS: would love to hear anyone's suggestions for labeling. at this point i'm looking at black ink on transparent paper, then polyurethane coating.

[TubeNoob]

Oh and by the way - the amp gets the credit and not the guitar - it's a knock-off strat i got for 25 euro that is literally called a "Fire Legend"! I am the king of crappy gear; probably why i love DIY so much - because it allows us poor folk to rise above!

[Chuck H]

Oh, I didn't say "nice guitar". I said "nice sounding guitar". My history is peppered with budget instruments that just felt and sounded "right" so I'd buy them if I was in the market.

And I wouldn't change a thing about the basic circuit yo have since it's sounding good now. Trust me, it's easy to get lost a few mods down the road and lose your way back. Your ear memory starts to get fuzzy and you're never again sure if you should have left well enough alone. You could ADD circuits though. Like switchable caps and resistors to reshape EQ for heavier clipped tones, etc.

You mentioned needing to add shielded cables to crank it. Is the amp unstable at higher gain settings?

[TubeNoob]

Hey Chuck,

Yeah, i must admit that crappy guitar does sound good. I have another strat-copy that's by Samick and it's really dark, but that makes it sort of unique. Different is good.

I am open to adding circuits that could get the gain higher, but i do have to fix this squealing first. I am guessing its from non-shielded cables on some of these controls. I wired everything long at first but now I have the faceplate done i can re-do those cables to be short (and shielded).

I did put the 100pf bright cap on a switch, but didn't hear any difference. I'll double check i didn't wire (the simplest of all the mods) wrong.

yeah, when i lift the tone stack, or go for the mid-range control setting on high it does start to squeal.

[Chuck H]

Since the unit wasn't initially designed to be stable as a guitar amp the layout may be less than ideal for stability at higher gain. The location of tubes, transformers, terminal strips, etc. could have you running leads in less than ideal paths. A couple of clear gut shot pics might allow for a forum member to ID changes in wire routing that might help with the stability issue.