Can I turn my broken valve jr into a cab for my other valve jr?

ecc83

4114Effects said:

I can get you a suitable OT for £10 plus postage. Supplier lists them as line matching with power handling of 10w, but I've used them in a few low wattage amps as a single-ended OT with great results. Multi-ratio windings so you can easily match them to any output valve. 

Let me know if you want one as I'll be grabbing a couple for myself soon. 

Are those transformers tested to 2kV or so primary to secondary and frame? A valve OPT has to stand DC HT plus the peak AC which can be a kV or more if the impedance is mismatched and the stage driven hard.

A 100V line traff only has to cope with 200V peak tops. IF you use one FFS make sure the secondary is well tied to chassis and the chassis earthed!  They are also not designed to cope with standing DC but I guess the consequent distortion appeals !

Dave.

4114Effects

ecc83 said:

4114Effects said:

I can get you a suitable OT for £10 plus postage. Supplier lists them as line matching with power handling of 10w, but I've used them in a few low wattage amps as a single-ended OT with great results. Multi-ratio windings so you can easily match them to any output valve. 

Let me know if you want one as I'll be grabbing a couple for myself soon. 

Are those transformers tested to 2kV or so primary to secondary and frame? A valve OPT has to stand DC HT plus the peak AC which can be a kV or more if the impedance is mismatched and the stage driven hard.

A 100V line traff only has to cope with 200V peak tops. IF you use one FFS make sure the secondary is well tied to chassis and the chassis earthed!  They are also not designed to cope with standing DC but I guess the consequent distortion appeals !

Dave.

Good point. I always earth the chassis - nylock nut and shakeproof washer on it's own ground point. Secondary is also earthed on any valve amp I build. But great advice. 

Can't speak for the voltage handling capabilities, all I can say is I've tested my amps by running flat out for a good period of time without any undue consequences. But I will email the seller and see if I can get some test specs.

ICBM

BigMonka said:

I've got a version1 VJ with the wrong impedance transformer that @crunchman described, @4114Effects ;do you think that the OT you can get would improve the amp? I'm thinking more to do with the sound rather than reliability as it won't get any critical use.

Has it had the other V1 mods, in particular correcting the wrong input impedance? This makes a far bigger difference to the sound than the OT.

If you're not sure what this is - remove R1(68K, far too low, causes muddy tone) and R7 (1M, reduces gain, unnecessary) and fit the 1M from R7 in the R1 position.

For even more gain remove R6 (1M) and replace it with the 68K from R1.

57Deluxe

...why don't you turn it into a SESSION BluesBaby?? The chassis is designed for your cab

http://www.guitar-bass.net/gear/award-session-bluesbaby-22-review/

BigMonka

ICBM said:

BigMonka said:

I've got a version1 VJ with the wrong impedance transformer that @crunchman described, @4114Effects ;do you think that the OT you can get would improve the amp? I'm thinking more to do with the sound rather than reliability as it won't get any critical use.

Has it had the other V1 mods, in particular correcting the wrong input impedance? This makes a far bigger difference to the sound than the OT.

If you're not sure what this is - remove R1(68K, far too low, causes muddy tone) and R7 (1M, reduces gain, unnecessary) and fit the 1M from R7 in the R1 position.

For even more gain remove R6 (1M) and replace it with the 68K from R1.

@ICBM not yet but it's currently in bits ready for me to do it. I knew it was important to get the right input impedance but I didn't realise it would have a significant effect on the sound, so thanks I'll do that first!
Would you recommend putting a different value in the R7 position, or is it completely unnecessary? 

I think there are also a couple of mods I've seen to reduce noise, like an extra capacitor on the power supply and adding a wire or two to move one of the connections off the PCB.

ICBM

BigMonka said:

@ICBM not yet but it's currently in bits ready for me to do it. I knew it was important to get the right input impedance but I didn't realise it would have a significant effect on the sound, so thanks I'll do that first!

Would you recommend putting a different value in the R7 position, or is it completely unnecessary?

Unnecessary. It just reduces the effective value of the volume pot from 1M to 500K. Not sure why they didn't just use a 500K pot...

The R1 change makes a huge difference - the stock 68K resistor is like using the 'low' input on a classic 2-input amp, or having a tone-sucking vintage pedal permanently in line.

BigMonka

ICBM said:

BigMonka said:

@ICBM not yet but it's currently in bits ready for me to do it. I knew it was important to get the right input impedance but I didn't realise it would have a significant effect on the sound, so thanks I'll do that first!

Would you recommend putting a different value in the R7 position, or is it completely unnecessary?

Unnecessary. It just reduces the effective value of the volume pot from 1M to 500K. Not sure why they didn't just use a 500K pot...

The R1 change makes a huge difference - the stock 68K resistor is like using the 'low' input on a classic 2-input amp, or having a tone-sucking vintage pedal permanently in line.

Cool, thanks. I'll make that the first mod I do then :-)

crunchman

BigMonka said:

ICBM said:

BigMonka said:

@ICBM not yet but it's currently in bits ready for me to do it. I knew it was important to get the right input impedance but I didn't realise it would have a significant effect on the sound, so thanks I'll do that first!

Would you recommend putting a different value in the R7 position, or is it completely unnecessary?

Unnecessary. It just reduces the effective value of the volume pot from 1M to 500K. Not sure why they didn't just use a 500K pot...

The R1 change makes a huge difference - the stock 68K resistor is like using the 'low' input on a classic 2-input amp, or having a tone-sucking vintage pedal permanently in line.

Cool, thanks. I'll make that the first mod I do then :-)

I seem to remember that was one of the vital mods with the V1 Valve Jr.

It's a long time ago so I can't remember everything I did.  I think I remember doing something to the power supply.  I'm pretty sure I've got one of the stock filter caps at home which I replaced with a bigger value.  After a valve rectifier you don't use a really big value for the first filter cap.  They copied the cap value from the Champ circuit but put a solid state rectifier in it so you can improve power supply filtering by using a bigger value cap.  There may have been something to drop the voltage on the power valve as well, as it ran that very hot.

There used to be whole websites dedicate to modding Valve Jrs.

I would say to anyone looking for a 5W amp that I think the VHT Special 6 is a much better sounding amp.

ecc83

"After a valve rectifier you don't use a really big value for the first filter cap.  They copied the cap value from the Champ circuit but put a solid state rectifier in it so you can improve power supply filtering by using a bigger value cap. "

Aha! C'Man. There is thread in 'another place' about a single ended Fender which has Silicon rects but also two 220Rs in series with each leg. I have opined that this is also just a straight copy/hangover from the valve rectifier which needed stoppers for its protection* . Others disagree, saying it is an attempt to introduce 'sag' . Personally, I would have though that by the time you had got a single OP valve to be that far out of 'class A' things would sound terrible?

Mind you, a bigger res' cap is always a good idea for SE amps..They hum!

*Although very few guitar amps that I have seen maps of HAVE anode stoppers for the rect? The data sheets for VRects give tables for transformer impedance and maximum res' caps. The Mullard Valve amp Construction book goes into great detail about this. Its is of course POSSIBLE that the gitamp mains traffs were VERY carefully designed so that they needed no external resistors?

Ooo! Look! A pig just flew past!

Dave.

Strangefan

@ecc83 dood you clearly know your Amps! It's just a shame I don't have a clue what any of the above means  

BigMonka

@Strangefan sorry for hijacking your thread!  Out of interest are you using the stock speakers in you collection of VJs? I was wondering about trying one of the 8" celestions in mine but haven't found one cheap enough to try yet.

Strangefan

@BigMonka no hijack at all I have upgraded both the valves and speakers, I got the celestion, and its a great improvement, I would def recommend it 

Strangefan

So currently I have the broken one as an extension cab until the new transformer comes, the reason I'm making an effort with these amps is because they sound so bloody good, the modded one is honestly as soon as some high end amps I have owned, they really are good. 
http://i.imgur.com/iiSNrTb.jpg

BigMonka

Strangefan said:

@BigMonka no hijack at all I have upgraded both the valves and speakers, I got the celestion, and its a great improvement, I would def recommend it 

Cool, is it the Celestion 8-15 you've got or the Super8, or another one?
Which mods has your modded one got? It looks like an EQ circuit or some description has been added?

Strangefan

BigMonka said:

Strangefan said:

@BigMonka no hijack at all I have upgraded both the valves and speakers, I got the celestion, and its a great improvement, I would def recommend it 

Cool, is it the Celestion 8-15 you've got or the Super8, or another one?
Which mods has your modded one got? It looks like an EQ circuit or some description has been added?









I swapped the chassis so the ga5 is now in the epi chassis, it's has a shed load done to it, triode /pentode switch, standby switch, 8/4 ohm switch, resistor and cap changes, I went for the 8/15 celestion, ehx preamp tube and stovak power tube, it honestly sounds fantastic, 
The best part o got the fully modded ga5 for 30 quid! 

http://i.imgur.com/1MpPBRv.png