Low gain preamp valves - recommendations/advice?

steamabacus

I've just tracked down some problems with my home studio Blackstar HT-5 to a preamp valve gone bad. It's a GE 5751 which I put in there to replace the stock Sovtek 12AX7 (or is it a 7025?). I've now put the stock Sovtek back in and all is well but I do prefer the response of a lower gain valve in the preamp.

So, I'm looking to buy a replacement. I'm looking at a 5751 again or maybe a 12AY7.

Any recommendations for good examples of either of these valves? Nothing too esoteric and pricey, though.

Also, I've been looking at HotRox but are there any other recommendations for valve suppliers? Any valve traders on the forum, maybe?

DartmoorHedgehog

I'm using an EHX 12AY7 in my HT-20.  Sounds good to me, and is the only new AY7 I could find.

ESchap

Try the EHX 12AY7/6027A, I've found it pretty close in tone to the old USA production valves, particularly after its got a few hours use under its belt.  

Remember with the 12AY7 you're dropping the gain a bit compared to a 5751 (40 versus 70 where 12AX7 is 100). 

JJ do a 5751 but I've not tried them.

steamabacus

Thanks for the recommendations.

I was looking at the EH 12AY7s on HotRox. They do the 'gold' version as well. Is there any difference other than gold plated pins?

I hadn't realised the 12AY7 was that much lower than the 5751. I may get one of each as I know the 5751 works well for me - a little less gain does appeal but 40 against 70 may be too much.

crunchman

Karltone are mentioned a lot on here as being very good.  They have JJ5751s listed.

Watford Valves have some NOS US made 5751s.  They should last longer so the the extra cost may work itself out long term.

Langrex are the other one I've seen mentioned on here.

DartmoorHedgehog

steamabacus said:

I hadn't realised the 12AY7 was that much lower than the 5751. I may get one of each as I know the 5751 works well for me - a little less gain does appeal but 40 against 70 may be too much.

I know the HT5 is a bit differently voiced, but with a 12AY7 in the drive channel of my HT20 it's still got far more gain than I ever use - just makes the lower gain end of the scale a bit more controllable.  Can't say I've tried it in an HT5 though.

ICBM

The difference in gain isn't as much as you might think - the full gain of each valve type is not used by the circuit normally, so a lower gain type doesn't reduce it as much - even though the two halves are in series in the Blackstar.

The 12AY7 is a nice valve for a clearer, brighter tone as well as slightly less gain.

I'm personally not keen on 5751s - every one I've tried (all old-production, from various different brands) has sounded too middy and oddly nasal, or too 'smooth' somehow. Given that I think JJ 12AX7s sound a bit like that anyway I don't think I'd like one of their 5751s!

You could also try a 12AT7 - often these don't sound great in 'gain' stages, but there are some that do, including the NOS JAN/Philips one Watford Valves are doing for the silly price of £10 each at the moment.

steamabacus

DartmoorHedgehog said:

steamabacus said:

I hadn't realised the 12AY7 was that much lower than the 5751. I may get one of each as I know the 5751 works well for me - a little less gain does appeal but 40 against 70 may be too much.

I know the HT5 is a bit differently voiced, but with a 12AY7 in the drive channel of my HT20 it's still got far more gain than I ever use - just makes the lower gain end of the scale a bit more controllable.  Can't say I've tried it in an HT5 though.

It was wanting control over the lower end of the gain knpb that led to me putting the 5751 in the HT-5 in the first place. It did help but it certainly wasn't a huge difference. These days, I mostly use my pedalboard into the clean channel - I often push it hard with a clean boost and it's here that I'm wanting a bit more headroom (without totally re-jigging how the boosts and drives on the board are set up).

ICBM said:

The difference in gain isn't as much as you might think - the full gain of each valve type is not used by the circuit normally, so a lower gain type doesn't reduce it as much - even though the two halves are in series in the Blackstar.

The 12AY7 is a nice valve for a clearer, brighter tone as well as slightly less gain.

I'm personally not keen on 5751s - every one I've tried (all old-production, from various different brands) has sounded too middy and oddly nasal, or too 'smooth' somehow. Given that I think JJ 12AX7s sound a bit like that anyway I don't think I'd like one of their 5751s!

You could also try a 12AT7 - often these don't sound great in 'gain' stages, but there are some that do, including the NOS JAN/Philips one Watford Valves are doing for the silly price of £10 each at the moment.

Yes, I've really noticed with putting the stock Sovtek 7025 back in that the EQ has changed considerably. I've had the amp set with the ISF way round to the 'UK' side and I've had to really push the treble to nearly max. With the stock valve it's brighter and has a very different midrange character.
You're making a 12AY7 sound like a good bet for what I'm after. I'm still thinking I'll get one of each (5751, 12AY7) and compare the two. I'll have a look at those 12AT7s too, though '£10' at Watford Valves usually ends up as nearer £20 with tax and delivery. 

ecc83

Being a bit cautious I have looked up the anode loads for the HT-5 pre amp and find they are rated at 0.5W. The second stage is biased on the "cold" side so I doubt that any of the valves mentioned would cause over dissipaton but it would be advisable IMO to check the anode at pin six for volts and do a quick calculation.

I found the 7025 to be a very reliable valve and much better for noise, hum and microphony than the Sovtek ECC83 bog s. Not that the HT-5 is prone to these problems.

 

Dave.

DartmoorHedgehog

@ecc83, just out of interest, does that apply to the HT20 as well?  Running a 12AY7 in the drive channel is a pretty common thing and generally accepted (unofficially) as safe.  I've been using mine with an AY for at least a couple of years with no ill effects noticed.  But is it technically outside specs for the amp and we're only getting away with it because it's built fairly generously? (I know 12AY7s draw more current than AX and AT)

steamabacus

ecc83 said:

Being a bit cautious I have looked up the anode loads for the HT-5 pre amp and find they are rated at 0.5W. The second stage is biased on the "cold" side so I doubt that any of the valves mentioned would cause over dissipaton but it would be advisable IMO to check the anode at pin six for volts and do a quick calculation.

I found the 7025 to be a very reliable valve and much better for noise, hum and microphony than the Sovtek ECC83 bog s. Not that the HT-5 is prone to these problems.

 

Dave.

Thanks for the info, Dave, although measuring pin voltages is outside my meagre abilities. When I fitted the 5751, I just trusted the description of it being a 'direct replacement' for an ECC83. The same with the 12AY7 - I've always seen them described as 'direct replacement' and therefore safe to try.

Would you care to explain a little more what this 'over dissipation' might be and what possible problems might result (in the layest of layman's terms, of course)?

ICBM

The 12AY7 and other 12A*7 types are *not* direct replacements for the 12AX7 that simply have lower gain - they are similar types which can *usually* be substituted in *most* amps.

But there are some important differences, some of which can make them a risk to either the valve or the amp, or possibly both.

The gain by itself matters because in a cathode-biased circuit (which almost all preamp valves are) a lower-gain valve will bias itself hotter and draw more current, which then produces a bigger voltage drop across the plate resistor and may exceed either its power rating or voltage rating (often overlooked).

There are three other ratings which can matter: absolute maximum voltage rating, valve plate dissipation and cathode-to-filament voltage rating.

The maximum voltage and plate dissipation generally only apply in things like reverb drivers which are fed from a high voltage and are essentially running as miniature power valves, but the cathode voltage rating is critical in many amps which use 'cathode follower' circuits - the 12AY and AT7 simply do not have a high enough rating to work safely in these positions, and if you try it the valve will probably fail, with a small risk of amp damage. (Even some modern 12AX7s aren't made to the correct spec and can fail like this.)

Cathode followers are common in Marshall-type amps and quite a few others, so be careful.

The only definitely safe substitution for a 12AX7 unless you know more about the circuit is the 5751, and even then it's just possible it could overload the plate resistor if the amp is poorly designed.

So you need to check first!

I'm pretty sure the HT pedal and amp circuit *is* safe - and definitely isn't a cathode-follower - so it should be OK here, but Dave knows more about it.

ecc83

You can see from ICBM's comprehensive, but by NO means complete list opf caveats that ad hoc valve sawpping is fraught!

The cathode follower (ecc83) is used in the A series and I indeed have had Sovteks go bad here (a punched thru cathode in an A100 is sound you remember!). Fortunately no "collateral" damage to the amps is caused.

The anode loads in the pedals are naturally rather wee! So stick with the right valve please! The HT range all use 1/2 or 0.6W types and should be ok for experiment..But that is NOT official!!

The Series ones use an ECC82 driver, anything else is likely to fail PDQ and sound ***t.

Dave.

steamabacus

Thanks for the info, guys.

DartmoorHedgehog

Thanks Dave,

12AY7 certainly has been behaving fine in my HT-20 for ages (although I did have one die recently, possibly unrelated), and sounds better for my tastes than a 12AX7.  I know it's not officially endorsed though

steamabacus

Well, I ended up ordering a couple of EH Gold valves from HotRox - a 5751 and a 6072A/12AY7.

I've just had a quick valve swapping session and, for my money, they are both an improvement on the stock 7025 (that I put back in after the GE 5751 went bad).

I started with the 5751 (as I had that in there before) and it definitely darkens the tone as well as dropping the gain.

I then tried the 6072A/12AY7 and it's noticeably lower gain than the 5751 but brighter. I like it and I've left this one in there. It allows me to hit the front end with my clean boost (which I often do for clean leads) with just a touch of grit when digging in hard which suits the way I'm set up.

It's a real pain trying to line up the pins (with the HT-5's sunken out of sight preamp valve base) and wriggling the valves in. It takes a little bit of force but too much will bend the pins if you're just a little out of skew. I found I had to gently readjust a few of the pins on the valves to make sure they were straight and true. I found lining the new valves's pins up against the pins of the valve that has just been taken out helped make sure the new valve would slip in without too much of a problem.

Be careful, though, if you're changing a preamp valve on an HT-5 for the first time. Patience is the key.

DartmoorHedgehog

Glad it worked out well - I like the 12AY7 in my HT20.

steamabacus said:

Be careful, though, if you're changing a preamp valve on an HT-5 for the first time. Patience is the key.

They're very recessed in the HT20 as well. I keep a piece of rubber cut from an old bike inner tube with my spare valves - makes it easier to get a grip on them to pull them out.

steamabacus

DartmoorHedgehog said:

Glad it worked out well - I like the 12AY7 in my HT20.

steamabacus said:

Be careful, though, if you're changing a preamp valve on an HT-5 for the first time. Patience is the key.

They're very recessed in the HT20 as well. I keep a piece of rubber cut from an old bike inner tube with my spare valves - makes it easier to get a grip on them to pull them out.

I used a wiggling, rotating action when pulling.

Another thing I forgot to mention is make sure you take a note of the orientation of the 'notch' in the pin layout when you take the old valve out (there's no way of looking in there without removing the chassis from the headshell). On my HT-5 the notch/gap was directly on the outside edge (on the left looking into the rear of the headshell) so it wasn't too hard to realign the new valve correctly. I also found I could rotate the valve slightly and 'feel' as the pins located into the valve base slots.