Epi Valve Junior mods - advice please? :)

[dannyart]

Hi everyone. I have one of the first Valve Jr combos, which I bought years ago, and have just recently decided to do the mods to it to get rid of that noise and get some more gain!... Had great fun with it, I have done the resistor mods to increace impedence and gain, the input jack, a tone control, and all the hum cancelling mods - big caps/bridge rectifier etc.

The thing is, now when I switch the amp on it takes about 2minutes to get any sound out of it. It used to get going in about 10secs - with that old familiar hum kicking in! Once it does get going it sounds awsome but there is still a very slight gritty hum that gets louder as I turn it up. Its the delay in getting started thats worrying me - its almost like its got an invisible standby switch... no power to the valves, no sound, no nothing, then after about 2mins it all kicks in and comes alive! I am at a loss to why this is happening?

Any help or suggestions are much appreciated... Cheers guys!

[Kazz]

sounds like a transformer issue to me.....but let's see what Duh Voodoo Man thinks as he is the local hot mod.

[duhvoodooman]

While I appreciate Kazz's comment, he gives me too much credit! Haven't run into this before and I'd be speculating on the cause. A transformer problem might be a possibility, but the first thing that comes to mind is that the tube filament heaters aren't getting the right voltage or current for some reason. I'd definitely check the bridge rectifier installation and make sure that looks good. That should be putting out something right around 6V or so to the board. I'd definitely check that with a multimeter.

Our best amp guy is Tunghaichuan. I'll shoot him a PM to check this thread & comment.

[tunghaichuan]

While I appreciate Kazz's comment, he gives me too much credit! Haven't run into this before and I'd be speculating on the cause. A transformer problem might be a possibility, but the first thing that comes to mind is that the tube filament heaters aren't getting the right voltage or current for some reason. I'd definitely check the bridge rectifier installation and make sure that looks good. That should be putting out something right around 6V or so to the board. I'd definitely check that with a multimeter.

Our best amp guy is Tunghaichuan. I'll shoot him a PM to check this thread & comment.

Well, DVM gives me too much credit. ;)

If the amp didn't have the delayed warmup before the mods then I'd speculate that something in the mods isn't right. The first thing I'd do is substitute a known-to-be-good set of tubes to rule out any tube weirdness. As DVM said you will need to check the voltages. Do you have a DMM? It could be the power transformer going bad. If you need to get a new one, I believe an eBay seller named britishaudioservice has them for sale on eBay resonably priced. Did you change the output transformer?

Another thing that might be causing problems is a flaky solder joint or joints. Check all of them and make sure that they are nice and shiny. A bad solder joint will be dull and pasty looking.

For this reason it is usually best to do one mod at a time to see how it affects the amp. That way you know precisely which mod is causing the problem. A little late now, I know but just some unsolicited advice for the future. :)

tung

[duhvoodooman]

Incidentally, dannyart, we like people who are new to The Fret to take the time to introduce themselves in our Fret Players forum (http://www.thefret.net/forumdisplay.php?f=13). Please stop be there and tell us a bit about yourself & your musical/guitar-related interests.

[dannyart]

Thanks for all the advice guys, really helpful! Yeah I'd planned to put a bit of info about myself on the forum, I'll do that today sometime...

I did the checks you said and it turns out one of the heater spade joints was cracked where it meets the board - probably due to me removing and replacing it too roughly. The fuse in the heater wire had also gone, and also the preamp valve didnt seem to be working properly! I replaced valve and fuse and soldered the heater wire direct to the next point in the circuit and its now working fine and warms up in about 10secs - job done! ...hopefully...

The thing is now, was it a coincidence that the valve died and the fuse blew or is it due to the mods Ive done? I've checked all my solder joints and stock joints etc and everything is ok. I cant see anything else that may be wrong. I suppose I'll just have to use it and see how it goes - sounds great though!

I've considered doing the full rewiring point-to-point with new components, but its quite expensive to source the exact parts over here - I may as well just buy a new head version instead! Incidently, how important is it to get the exact voltage right with caps in the VJ? i.e. will a 35v elect. work where a 25v used to be? ..or a 250v poly where a 400v was?

cheers :D

[duhvoodooman]

Excellent! Glad to hear that you got that sorted out. If it behaves properly, sounds good, seems stable and doesn't smell funny, I'd just keep playing it for a while! Hopefully you're all set on the amp now.

Re: the caps question--it's okay to go up on the V rating, but I wouldn't go down, especially on the high voltage stuff. There are DC voltages well over 300V on that board, so I definitely wouldn't substitute a 250V cap for a 400V!

And thanks for intro'ing yourself over at The Players forum--we appreciate that. :AOK:

[dannyart]

All ok with the VJ so far so looks like its all fixed and correct! Thanks for the advice about caps DVM.

One other mod I was planning to do was install a master volume or extra preamp valve so as to coax some serious overdrive out of the VJ at lower volumes. I have a few questions on this, if any of you have any more good advice that would be great...

1. Can a master volume be installed without any other mods, & where would it go ..inbetween the two valves somewhere??

2. If an extra preamp valve was installed would it just be a mirror of V1 with power, heaters etc all tapped off the same points as V1 and placed inbetween the two existing valves?

3. Would either of the above give the VJ that fizzy/buzzy overdrive that most of us hate? I'd wondered if the above mods would basically turn the VJ into a ValveSpecial, which I demo'd and didnt like at all - so is it worth doing basically.

Cheers for the help again guys!

[just strum]

If it behaves properly, sounds good, seems stable and doesn't smell funny...

Hey back in my younger days that was the same criteria I used to select a date.

[tunghaichuan]

One other mod I was planning to do was install a master volume or extra preamp valve so as to coax some serious overdrive out of the VJ at lower volumes. I have a few questions on this, if any of you have any more good advice that would be great...

1. Can a master volume be installed without any other mods, & where would it go ..inbetween the two valves somewhere??


You can install a MV without any other mods. One way to do it would be substitue resistor R5 (220k ohms) with a 250k audio pot. In my experience, this does not sound all that good. I usually end up turning the pot all the way up and it becomes a case of "might as well leave the original resistor installed."



2. If an extra preamp valve was installed would it just be a mirror of V1 with power, heaters etc all tapped off the same points as V1 and placed inbetween the two existing valves?


What you would need to do is install another node on the power supply rail, that is a decoupling resistor between R13 and a new 22uF/450v capacitor. If you run four gain stages (two 12AX7s) off of the same node on the power supply rail you can get into intermodulation distortion problems.

Realistically, this type of mod is hard to do without lots of experience. Generally, adding two gain stages is a recipe for squeal. You have so much gain that you have to pad it down with voltage dividers to get the gain under control. Also there is the issue of adding the other tube and connecting it into the PCB, which is cramped and wasn't designed with additions to it in mind.



3. Would either of the above give the VJ that fizzy/buzzy overdrive that most of us hate? I'd wondered if the above mods would basically turn the VJ into a ValveSpecial, which I demo'd and didnt like at all - so is it worth doing basically.


Probably, the MV sounds buzzy at low volumes. Tacking on a 12AX7 to the front end may sound buzzy without the proper cathode/anode resistor and capacitor values.

Here is another option to consider:

Installing the VVR (http://yeomansinstruments.blogspot.com/2008/07/vvr-greatest-thing-ever.html)

This mod allows you reduce the B+ voltages in the amp which allows it to overdrive quicker and at low volumes. I have not tried this yet, but by all accounts, it sounds very good.

Another option is what I have done: I did a set of basic mods to my VJ to rebias the preamp and power amp tubes. Beefed up the power supply, change out all the crummy hardware that comes stock on the amp, and installed a 5751 tube for the 12AX7 and a JJ EL84 for the stock Sovtek EL84. I run the amp relatively clean and use pedals for OD.

tung

[dannyart]

Thanks for that Tung, the vvr sounds good! Do you know if I can use any mosfet for this or does it have to be NTE2377? Not too clued up on these, or how they work, to be honest :confused:

[tunghaichuan]

Thanks for that Tung, the vvr sounds good! Do you know if I can use any mosfet for this or does it have to be NTE2377? Not too clued up on these, or how they work, to be honest :confused:

You have to use the NTE2377 or equivalent. Those are high voltage, high current mosfets and have to be as they dissipate lots of power. A mosfet with lower ratings will burn out quickly in that circuit. In the States those NTE2377s are about $11 each. :(

Here's a link to the data sheet:

http://www.nteinc.com/specs/2300to2399/pdf/nte2377.pdf

Will take 900v, dissipates 150W.

tung

[dannyart]

Cheers Tung, that makes more sense now. I had a quick look at mosfets on Wiki too. I cant find the NTE one here in the UK but I found IXFH10N90-ND, which appears to have similar spec, but no one has that in stock either! Looks like I'll be trying something else. Thanks for the advice though.

One other thing I had a quick look at was an L-pad. Looks like a dead simple circuit to put together, and if used with a switch I could go from full to half to quarter power etc, depending on the two resistors. Have you any experience of using them... do they adversly affect the tone like the master vol?

[tunghaichuan]

One other thing I had a quick look at was an L-pad. Looks like a dead simple circuit to put together, and if used with a switch I could go from full to half to quarter power etc, depending on the two resistors. Have you any experience of using them... do they adversly affect the tone like the master vol?

I don't have any experience with an L-pad, but I know of a few people who have used them. They are of the purely resistive load type. Some people like them as they don't compress like some of the attenuators with inductors like the Weber MiniMass and the THD Hotplate. My Weber MiniMass compresses the signal quite a bit at the highest attenuation settings.

One thing to keep in mind about all attenuator circuits: they are deceptive because even though there isn't much sound coming out of the amp, the amp is still putting out full power. So tubes will wear out just as fast as if you had full volume coming out of the amp.

tung

[dannyart]

Help needed!!... Hi again chaps. Been away on a shoot for a few days, got back the other day and had a chance to have a jam with some friends so I thought I'd take the VJ and really crank it to see how it sounds since I've done all the mods.

I havent had a chance to get the volume past 12oclock in the house so far cause its now a lot louder, anyway, in the practice room I turned it up past 2oclock, then 3, then all the way full, and it started making this bizzare tremolo noise! ...the volume was wooshing in and out all by itself, at about half second intervals, exactly like a tremolo, and was getting harsher and harsher, it sounded like it was gonna blow, so I turned it back down and it went back to normal!:confused:

Basically I cant get the volume past 2oclock without it doing this, which is pretty annoying. Short of removing all the mods I've done and working out the cause by process of elimination, does anyone know what could be causing this and how to fix it?? Ive never encountered anything like it before.

Cheers guys

[ShortBuSX]

Seems like it could be a couple of minor things.
With my Deville I noticed if I had it too close to a wall(or corner) and had the volume up a decent level the sound would oscillate...I gave it some distance and it stopped.

I dont know much about the VJ, but how many power tubes does it have?
If it has 2, could one of them drifted out of being "matched" and cause this?

Could also be a preamp tube(or power tube) thats gone microphonic...with the amp turned on, tap on each of the tubes, if you can hear this through the amp youve found you culprit.

I dunno if any of these will actually help, but I figured Id give you some ideas that didnt consist of ripping your mods apart.

[tunghaichuan]

For a start, what mods have you done to the amp?

What you're describing sounds very weird, not normal at all. Let us know what mods you've done and then we can go from there.

tung



Help needed!!... Hi again chaps. Been away on a shoot for a few days, got back the other day and had a chance to have a jam with some friends so I thought I'd take the VJ and really crank it to see how it sounds since I've done all the mods.

I havent had a chance to get the volume past 12oclock in the house so far cause its now a lot louder, anyway, in the practice room I turned it up past 2oclock, then 3, then all the way full, and it started making this bizzare tremolo noise! ...the volume was wooshing in and out all by itself, at about half second intervals, exactly like a tremolo, and was getting harsher and harsher, it sounded like it was gonna blow, so I turned it back down and it went back to normal!:confused:

Basically I cant get the volume past 2oclock without it doing this, which is pretty annoying. Short of removing all the mods I've done and working out the cause by process of elimination, does anyone know what could be causing this and how to fix it?? Ive never encountered anything like it before.

Cheers guys

[Andy]

possibly the power tube going bad ?

[dannyart]

Hi guys. I've tried 3 different sets of tubes in it (sovteks, and jjs) and it still does it, so it cant be that. It even does it without a lead plugged in. Basically it didnt do it at all before I did the mods, here are the ones I've done:

Nylon input jack
Higher quality tone caps - C1 & C2
Change R1 & R2 resistors for the high imp
Added a 100uf 450V across C6
Added bridge rectifier to convert to DC

Got most of these mods from this site, the S2amps site and another one I cant remember at the mo. Im gonna start removing some of them later and see what happens. Any ideas chaps?

[duhvoodooman]

OK, before you go ripping stuff out, let's work the problem for a while. None of those mods would seem to be an obvious cause of the behavior you're describing. There's always the possibility that one of them exacerbated an existing problem in the amp. Couple of questions/requests:


Can you provide some additional detail on the individual mods? What were the resistor and cap ratings of the changes you made? Cap types? What cap rating did you install across the DC leads of the rectifier?
Had you ever run the volume way up on the amp before doing the mods, to be sure the amp didn't have this issue previously? Sorry if that sounds elementary, but I'm thinking of my own situation here. I never used to "dime" the volume on my own VJr because I play it in a small room, and it was just too loud. I've only started doing this in the past few months, using a resistive load attenuator.
Is it possible for you to take some good, clear close-up photos of your mods and post them here for perusal? Sometimes something jumps out of a photo when viewed by others that you may be overlooking or not recognize as a problem yourself.

[dannyart]

I found the fault!... It was a mod that I'd forgot to mention before - replacing R5. I got this mod from the S2 site which states: "You can increase
power tube overdrive by increasing this resistor up to 1M" ...which is what I'm after. I replaced it with a 470K just to try it & it didnt do anything so I just left it in - and because it only had an effect with the volume past 2oclock I didnt notice it for a while. The reason I forgot it is because I used a 3band resistor identical to the stock ones - where as the other resistors I replaced are blue 4band ones - different manufacturers I guess.

Anyway, I put back the stock 220K and it all works great again now, at all volumes! I have had it up to 10 in the practice room previous to doing the mods, but now its just amazing!

I've got an old Orange cab with a Celestion Alnico Blue in it so I might try wiring up an output jack for the 8ohm output and try it through that next!

Thanks for the help and ideas again guys! Incidently, does anyone know why the 470K resistor had that effect?

[tunghaichuan]

I found the fault!... It was a mod that I'd forgot to mention before - replacing R5. I got this mod from the S2 site which states: "You can increase
power tube overdrive by increasing this resistor up to 1M" ...

Thanks for the help and ideas again guys! Incidently, does anyone know why the 470K resistor had that effect?

That is interesting. The 470k resistor should not have had that effect. The data sheet for an EL84 says you can use up to 1meg in that position for a cathode bias amp. The only thing I can think of is that the tube you have in there just didn't like that value.

tung