Looking for 6EU7 valves

knuckleberryfinn

I'm struggling to get hold of 6EU7 valves - does anyone know of a place that stocks them?  (Hotrox only have Sovteks, and they are £20 each, which I don't think is a very good deal.)  Or does anyone have a few they'd like to re-home?

andy1839

I pulled about 30 valves from a number of old reel to reel machines last weekend.
All Mullard or GEC, I'll check at the weekend to see what I've got, I didn't have time to research them all. There were only two 12AX7's and one EF86, the rest I didn't recognise and had odd designations. 

knuckleberryfinn

andy1839 said:

I pulled about 30 valves from a number of old reel to reel machines last weekend.
All Mullard or GEC, I'll check at the weekend to see what I've got, I didn't have time to research them all. There were only two 12AX7's and one EF86, the rest I didn't recognise and had odd designations. 

@andy1839 Cheers - I'll keep my fingers crossed.

andy1839

Hi mate, sorry for the late reply, I've had a look and the closest I've got is a 6AU6.  The rest of them I've never heard of! 

RiftAmps

I have some NOS EICO (Mullard made) in storage somewhere, probably around £25 each.

ecc83

Pinouts are totally different but the electrical characteristics look virtually identical to the ECC83/12AX7? I would use a B9A plug and valveholder and make an adaptor, hold the valve in a tool clip.

Dave.

ecc83

http://www.ebay.co.uk/itm/1pc-Gold-12AX7-TO-6EU7-tube-converter-adapter-Brass-metal-case-/191812104990?tfrom=201123876948&tpos=top&ttype=price&talgo=undefined

Well! Who would have thought it! ^

Dave.

ICBM

Why is £20 not a good deal for a fairly rare specialist valve type?

Not trying to be funny!

knuckleberryfinn

ICBM said:

Why is £20 not a good deal for a fairly rare specialist valve type?

Not trying to be funny!

I seem to remember them being a lot cheaper a few years ago. As ecc83s are half the price, I think I might just rewire the sockets - I was only experimenting with reducing noise (made no difference at all). Or maybe I'll get those adaptors Dave suggested - cheers Dave.

ICBM

Rewiring the sockets is the best plan if it's not a PCB amp with board-mounted ones. Even those adaptors will cost over £20 when you include the cost of a 12AX7. (Although you probably already have those.)

To be honest I don't think valves have ever been cheaper than they are now, in real terms. You can get a perfectly decent 12AX7 for under a tenner, which is actually less in plain numbers than they were 20 years ago, let alone accounting for inflation.

Aidanspaghetti

Ive just bought a batch of u31's and 6rby's for my old Selmer..
There's good deals to be had on old tubes on ebay if you look around

ecc83

ICBM said:

Rewiring the sockets is the best plan if it's not a PCB amp with board-mounted ones. Even those adaptors will cost over £20 when you include the cost of a 12AX7. (Although you probably already have those.)

To be honest I don't think valves have ever been cheaper than they are now, in real terms. You can get a perfectly decent 12AX7 for under a tenner, which is actually less in plain numbers than they were 20 years ago, let alone accounting for inflation.

Cheap, yes. I recall testing valves for customers because they were costly. They brought them in out of their radio or (worse!) telly and I had the job of running the Avo tester. I got so I remembered the numbers for the Paxolin cards for all the common types, EF80, ECL80,ECH33,6V6 etc. Cost them sixpence a pop (2.5p) but that was waived if they bought a new valve.

Of course! When they got home they would invariably fit them back in the wrong positions (despite there almost always be a diagram in the radio/tv . Don't get THOSE these days!) blow the bugger up and come back with it next day!

Jeeez! The STUFF I could/should have salted away!!

Dave.

DJH83004

I have to agree with ICBM on this one considering the rarity of the 6EU7 I amazed any main stream manufacturer is still producing them, suspect the price is justified. Just out of interest regarding the revised pin layout RCA quote 'Noise and hum are minimised by double wound helical heaters and a new base layout which keeps heater loads well away from the grid leads'  so there you have it 

ICBM

DJH83004 said:

I have to agree with ICBM on this one considering the rarity of the 6EU7 I amazed any main stream manufacturer is still producing them, suspect the price is justified. Just out of interest regarding the revised pin layout RCA quote 'Noise and hum are minimised by double wound helical heaters and a new base layout which keeps heater loads well away from the grid leads'  so there you have it 

Nowadays amp companies just solve that by DC heating the filaments… although I'm not really a fan since the rectifier circuits are a fairly common cause of failures. (Not just in Marshalls!)

knuckleberryfinn

The preamp is hissing like a mutha anyway (can't find a solution and I've tried everything) so who cares about a bit more hum?

jpfamps

knuckleberryfinn said:

The preamp is hissing like a mutha anyway (can't find a solution and I've tried everything) so who cares about a bit more hum?

Have you changed the anode load resistor? That can be a significant source of noise.

Before you do that, you can short the valves control grid to ground to see where the noise if being injected.

knuckleberryfinn

jpfamps said:

knuckleberryfinn said:

The preamp is hissing like a mutha anyway (can't find a solution and I've tried everything) so who cares about a bit more hum?

Have you changed the anode load resistor? That can be a significant source of noise.

Before you do that, you can short the valves control grid to ground to see where the noise if being injected.

Definitely V1 (it's almost silent when I remove the valve), but can't remember which side.  I've changed all the resistors associated with V1 (for metal film), as well as the main B+ feed resistors.  I've done loads of other stuff too - but it was over a year ago now, and I can't remember what.  This happened in another blackface build, and the only solution I could come up with was bleeding treble to ground through a 10pf cap (I think that was the value).  It made things duller, obviously, but there was plenty of treble on tap.  Having said that, there's a lot of gain on tap, so maybe I could reduce hiss by injecting negative feedback into the preamp. 

ESchap

knuckleberryfinn said:

jpfamps said:

knuckleberryfinn said:

The preamp is hissing like a mutha anyway (can't find a solution and I've tried everything) so who cares about a bit more hum?

Have you changed the anode load resistor? That can be a significant source of noise.

Before you do that, you can short the valves control grid to ground to see where the noise if being injected.

Definitely V1 (it's almost silent when I remove the valve), but can't remember which side.  I've changed all the resistors associated with V1 (for metal film), as well as the main B+ feed resistors.  I've done loads of other stuff too - but it was over a year ago now, and I can't remember what.  This happened in another blackface build, and the only solution I could come up with was bleeding treble to ground through a 10pf cap (I think that was the value).  It made things duller, obviously, but there was plenty of treble on tap.  Having said that, there's a lot of gain on tap, so maybe I could reduce hiss by injecting negative feedback into the preamp. 

Did you try changing the V1 grid stoppers (68k?)  I had similar problems with an amp very hissy, changed all the usual suspects.  Didn't think that those resistors would have an impact, read that they can and reducing the value can help, changed them to 20k .... no more hiss (well, very little) and still no RF with that value.

jpfamps

ESchap said:

knuckleberryfinn said:

jpfamps said:

knuckleberryfinn said:

The preamp is hissing like a mutha anyway (can't find a solution and I've tried everything) so who cares about a bit more hum?

Have you changed the anode load resistor? That can be a significant source of noise.

Before you do that, you can short the valves control grid to ground to see where the noise if being injected.

Definitely V1 (it's almost silent when I remove the valve), but can't remember which side.  I've changed all the resistors associated with V1 (for metal film), as well as the main B+ feed resistors.  I've done loads of other stuff too - but it was over a year ago now, and I can't remember what.  This happened in another blackface build, and the only solution I could come up with was bleeding treble to ground through a 10pf cap (I think that was the value).  It made things duller, obviously, but there was plenty of treble on tap.  Having said that, there's a lot of gain on tap, so maybe I could reduce hiss by injecting negative feedback into the preamp. 

Did you try changing the V1 grid stoppers (68k?)  I had similar problems with an amp very hissy, changed all the usual suspects.  Didn't think that those resistors would have an impact, read that they can and reducing the value can help, changed them to 20k .... no more hiss (well, very little) and still no RF with that value.

All resistors generate Johnson noise. The higher their value the more noise they produce.

Reducing grid stopper will reduce noise.

If RF breakthrough is a problem then a small cap can be put on the input.

You can easily identify the contribution of noise from the grid stoppers by shorting the control grid to ground an listening to how much the noise goes down.

ESchap

Sorry OP de-railing a little .... @JPFamps ; in your experience, is there any benefit to having the V1 grid stoppers on the valve base rather than at the input jacks?  I've seen them in both locations.

jpfamps

ESchap said:

Sorry OP de-railing a little .... @JPFamps ;; in your experience, is there any benefit to having the V1 grid stoppers on the valve base rather than at the input jacks?  I've seen them in both locations.

Yes.

They should be on the valve pins. 

Part of the reason for adding the grid stops is to isolate the input capacitance of the valves from the lead inductance.

The combined lead inductance and input capacitance can form a resonant circuit that can pick up RF.

Here's a practical example of the effect of placement of grid stoppers:

http://jpfamps.com/fender-tonemaster-tone-master-guitar-amp-repair/