SORTED: Dr Z Amp Blowing Fuses - Preamp To Blame. Safe to test??

freakboy1610

Have you tried the Z Talk forum? Very helpful people over there.
http://ztalk.proboards.com/thread/11503/blown-fuses-bad-rectifiers

jellybelly

I did have a look at that thanks @freakboy1610 ;;;

UPDATE:
So I took a chance and tried again, this time with 2 Tung Sol 12AX7s from my MJW and a Phillips 12at7 I had knocking around, plus what looked like the NOS 12at7 reverb driver already in there - the reverb was particularly quiet (and beautiful!) so I took a chance that it wasn’t the culprit and was the least likely to be ‘bad’.

I first checked again that the power valves and rectifier would work without the fuse going - sure enough it did - I saw a couple of arcs over the 2-3 mins I left it on for though. Took @ICBM’s advice and threw the standby switch first rather than let it ‘warm up’ - my understanding was that a valve rectifier made a standby switch redundant anyway but I might not be understanding correctly.

Anyway, turned off and put the preamp valves in, powered on with standby switch first and it’s working fine!! The hum is still there at higher ‘post’ settings but hopefully @hywelg can help there! I still suspect the rectifier however, so have ordered another (JJ), plus two NOS Mullard 12at7s and some RI Tung Sols. Hopefully I can have a bit of confidence in it then!!

The valves I took out got checked via my MJW V6 - the Tung Sol 12AX7 was gone, passed nothing, and the Fender-labelled 12at7 was noisy. The Sovtek 12AX7 was ok though so that’s something! I massively appreciate everyone’s input and help, hoping I can get this to a point I’d be confident gigging it, and perhaps a belated NAD once I’ve put it through it’s paces.
incidentally, it sounds sublime, like a Super Reverb and a blackface Bassman had a baby whose main source of sustenance was overdrive pedals. I’m excited to play it out!

jpfamps

Valve heater voltages are 6.3 V +/- 10%, so 6.9 VAC is fine.

Too low voltage will shorten valve life more dramatically than too high voltage, so I expect designers err on the side of too high voltage.

From RCA receiving tube manual:

http://members.iinet.net.au/~jamesrr/Heater%20voltage%20affecting%20valve%20life.gif

It's more difficult to get the filaments accurate in practice than you might think as there are only a relatively low number of turns on the secondary, so 5% or less accuracy is a good as you are going to get.

ecc83

Modulus_Amps said:

ecc83 said:

Modulus_Amps said:

Not impressed.

https://en.audiofanzine.com/tube-guitar-amp-head/dr-z-amplification/5-watts-mini-avec-compresseur/medias/pictures/a.play,m.1575542.html

Wow, that photo, how much does a rubber grommet cost, that would not pass CE testing in any test house in Europe.

They are probably designed for 230v +-10%.... then we have near 250v in some places in the UK, valve killers.

To be fair some amps are just known for killing valves fast... like some Cornfords

Err? 230 + 10% IS 253! But yes, stuff SHOULD be long term tested to withstand max mains input but sadly it seems not a lot is.

It is these sorts of tests and not using 'just' adequately rated components that causes some (cough!) amplifiers to be a bit more expensive than others. I do hope the changes I have heard about do not cause a  lowering of these standards at my old firm.

Dave.

 Yes but valve amp transformers don't work like that, +-10% on the primary gives you +-10% on the secondary, so our 6.3v heaters  could easily be near 7V in operation in some areas and that does not take account of how accurate the power transformer is operating.

If the amp was a European voltage amp - designed around 220v input - then when brought here it would be even worse... I know you know this.

My apologies M.A. I took a slightly wrong impression from your original post.

Dave.

ICBM

jpfamps said:

Valve heater voltages are 6.3 V +/- 10%, so 6.9 VAC is fine.

Too low voltage will shorten valve life more dramatically than too high voltage, so I expect designers err on the side of too high voltage.

From RCA receiving tube manual:

http://members.iinet.net.au/~jamesrr/Heater%20voltage%20affecting%20valve%20life.gif

It's more difficult to get the filaments accurate in practice than you might think as there are only a relatively low number of turns on the secondary, so 5% or less accuracy is a good as you are going to get.

It's interesting to see how drastically the lifespan falls off at less than 90% of the spec voltage.

This should be a concern to owners of Mesa amps with the 'Tweed' or 'Spongy' power settings, which run the whole PT at 15% under voltage, from memory. The idea is to get lower power and a more compressed feel, but I've always thought it seemed like a crude way of doing it, rather than dropping only the HT. I never particularly liked the sound anyway...

I generally run the late-70s Fender 'Ultra Linear' amps at the 260V setting rather than 240 and never noticed unusual numbers of valve failures, but that's less than a 10% reduction.

frictionfraction

Ive had a number of dr zs as I love the tone but I did find them prone to valve issues particularly rectifier valves causing blown fuses.  Changing the rectifier might well solve your problem but I would recommend you get it fully serviced.  Essex Amps in Witham know Zs inside and out and are absolutely top notch guys. Might cos a few quid but in my experience welll worth the cost if you plan to keep the amp. 

ICBM

frictionfraction said:

Ive had a number of dr zs as I love the tone but I did find them prone to valve issues particularly rectifier valves causing blown fuses.

All the Dr Z schematics I can find online for valve-rectified amps have the standby switch in the wrong place, so this is not surprising.

frictionfraction said:

Changing the rectifier might well solve your problem but I would recommend you get it fully serviced.  Essex Amps in Witham know Zs inside and out and are absolutely top notch guys. Might cos a few quid but in my experience welll worth the cost if you plan to keep the amp. 

Changing the position of the standby switch or adding a cap upstream of it would be much more effective and should stop the new valve blowing.

jellybelly

@frictionfraction  I’m afraid I’m a little north of Essex mate! Would be taking it to Martin at MJW if he was currently taking work but otherwise I think Roland Lumby has a good rep up here in Manchester - want to get that hum sorted and a once over just to be sure, along with a possible mod to the standby switch as @ICBM suggests. For now I’ll just leave the standby switch on and let it warm up the way god intended. 

Modulus_Amps

jpfamps said:

Valve heater voltages are 6.3 V +/- 10%, so 6.9 VAC is fine.

Too low voltage will shorten valve life more dramatically than too high voltage, so I expect designers err on the side of too high voltage.

From RCA receiving tube manual:

http://members.iinet.net.au/~jamesrr/Heater%20voltage%20affecting%20valve%20life.gif

It's more difficult to get the filaments accurate in practice than you might think as there are only a relatively low number of turns on the secondary, so 5% or less accuracy is a good as you are going to get.

Thats an interesting chart, would be more telling if the lifetime axis was linear, to me it looks like valve life is halved when heater voltage is at +-10% limits

Modulus_Amps

jellybelly said:

I did have a look at that thanks @freakboy1610 ;;;

UPDATE:
.

I first checked again that the power valves and rectifier would work without the fuse going - sure enough it did - I saw a couple of arcs over the 2-3 mins I left it on for though. Took @ICBM’s advice and threw the standby switch first rather than let it ‘warm up’ - my understanding was that a valve rectifier made a standby switch redundant anyway but I might not be understanding correctly.

Just leave the standby switch in the on position forever, or get it rewired. I did one recently and took it off the high voltage line now it just shunts signal to ground, will last forever and work the same from the users point of view.

Roland is a great tech, there is also Dan at DWJ in Liverpool that is pretty good and makes some lovely sounding amps

jpfamps

ICBM said:

jpfamps said:

Valve heater voltages are 6.3 V +/- 10%, so 6.9 VAC is fine.

Too low voltage will shorten valve life more dramatically than too high voltage, so I expect designers err on the side of too high voltage.

From RCA receiving tube manual:

http://members.iinet.net.au/~jamesrr/Heater%20voltage%20affecting%20valve%20life.gif

It's more difficult to get the filaments accurate in practice than you might think as there are only a relatively low number of turns on the secondary, so 5% or less accuracy is a good as you are going to get.

It's interesting to see how drastically the lifespan falls off at less than 90% of the spec voltage.

This should be a concern to owners of Mesa amps with the 'Tweed' or 'Spongy' power settings, which run the whole PT at 15% under voltage, from memory. The idea is to get lower power and a more compressed feel, but I've always thought it seemed like a crude way of doing it, rather than dropping only the HT. I never particularly liked the sound anyway...

I generally run the late-70s Fender 'Ultra Linear' amps at the 260V setting rather than 240 and never noticed unusual numbers of valve failures, but that's less than a 10% reduction.

I wire them to 260 VAC too, but the heater voltage usually comes out around 6.3 VAC as it's usually higher than this when set to 240.

Regardless, the relationship between mains voltage and filament voltage will be more complex than a linear relationship as the loading down of the filament supply will depend on filament current draw, which of course depends on filament temperature.

TimmyO

My (Marshall style) 18 watter blew fuses - new (NOS) rectifier valve cured that. 

re the harsh/unforgiving nature of the Maz, that's a lot of people's view and was how I felt initially - I owned my Maz 18NR for years and came to love its directness - it certainly tightens up your playing! - and I found it to be a particularly good amp for pedals - didn't come across the "ooh, not with this amp" thing you sometimes otherwise get

SunDevil

The noise with the post high is a separate issue - I had the same issue and got the same fix as @ ;

It transformed the amp

It involved 3 simple mods and 2 mins soldering

1) Check the Reverb Transformer is properly grounded
2) Extend the ground Bus for the pots to link the input jack
3) More the input cable from the jack to V1 across the board

ICBM

Modulus_Amps said:

Thats an interesting chart, would be more telling if the lifetime axis was linear, to me it looks like valve life is halved when heater voltage is at +-10% limits

No, log base3 - each division is a factor of three roughly. It looks like 90% voltage drops the lifetime from 5000 hours to about 1200 hours… whereas 110% only cuts it to about 2500.

Modulus_Amps said:

Just leave the standby switch in the on position forever, or get it rewired. I did one recently and took it off the high voltage line now it just shunts signal to ground, will last forever and work the same from the users point of view.

I don't like that as a standby mechanism because it's no use for troubleshooting, and is a dangerous trap for anyone working on the amp who may not realise that's all it does.

Much better to put it in the correct HT position - after the first filter cap but before the OT feed, as Leo Fender did… 

(In my opinion.)

jellybelly

SunDevil said:

The noise with the post high is a separate issue - I had the same issue and got the same fix as @ ;

It transformed the amp

It involved 3 simple mods and 2 mins soldering

1) Check the Reverb Transformer is properly grounded
2) Extend the ground Bus for the pots to link the input jack
3) More the input cable from the jack to V1 across the board

Hi mate hopefully it’s the same thing! May well be in touch to check a couple of things!!

Modulus_Amps

ICBM said:

Modulus_Amps said:

Just leave the standby switch in the on position forever, or get it rewired. I did one recently and took it off the high voltage line now it just shunts signal to ground, will last forever and work the same from the users point of view.

I don't like that as a standby mechanism because it's no use for troubleshooting, and is a dangerous trap for anyone working on the amp who may not realise that's all it does.

Much better to put it in the correct HT position - after the first filter cap but before the OT feed, as Leo Fender did… 

(In my opinion.)

Regardless of where it goes, if you have the amp open and are working on it you should know what you are doing, removing the HT fuse should kill all high DC voltage in the amp.

Modulus_Amps

ICBM said:

Modulus_Amps said:

Thats an interesting chart, would be more telling if the lifetime axis was linear, to me it looks like valve life is halved when heater voltage is at +-10% limits

No, log base3 - each division is a factor of three roughly. It looks like 90% voltage drops the lifetime from 5000 hours to about 1200 hours… whereas 110% only cuts it to about 2500.

Only cuts it to 2500 from what - about 6000 hours peak?? that is still half or below half of the optimal performance... according to that graph.

ICBM

Modulus_Amps said:

Regardless of where it goes, if you have the amp open and are working on it you should know what you are doing, removing the HT fuse should kill all high DC voltage in the amp.

I don't disagree that you should know what you're doing, but it nevertheless is a trap for the unwary given that at least 90% of amps do use the standby to turn off the HT. And it stops you safely hot-swapping valves by just putting the amp on standby rather than fully powering down. Yes, you can pull the HT fuse - but it's more of a faff.

I also just don't like it because it's deliberately pointless - Matamp do it, and it's almost like saying "we don't believe in standby switches, but if you insist on having one then rather than do it properly, we'll just not use it to do what it's supposed to."

I agree that standby switches aren't really necessary, but if you're going to have one then it should do the right thing.

Modulus_Amps said:

Only cuts it to 2500 from what - about 6000 hours peak?? that is still half or below half of the optimal performance... according to that graph.

Yes, that's what it looks like to me - 5 or 6000 hours at best, down to about 2500 at 110% voltage. That's probably not going to be noticeable in a guitar amp where lifespan is more normally down in the hundreds of hours due to either vibration or being run at well over the ratings.

ecc83

Ref filaments and OVER voltage? I was once tasked to try find out what would cause the 'Purple Death' to KT88s.  One of the indignities I subjected them to was excess heater voltage. I first thought 12, certainly 18 volts or so would pop them? Not a bit of it and, IIRC, I needed some 40 volts or so to do the dastardly deed. Fortunately I had a big, f'off,  V old 24-24 V traff out of a 200W 100V line PA  amp. 8x TO3 OP transistors. MONO!

Dave.

jellybelly

From Humbucker Music’s FAQ page (they’re a big Dr Z dealer in the US):

“I'm a bit confused about the Standby Switch.  Exactly when do I use it?

Many people still don't know how and when to use the stand by, and it's completely understandable since just about everyone has a different opinion on how to use it.  Well, in the Dr. Z Official Forum, Dr. Z himself confirmed the following for use on his amps:  

• Always check to make sure the amp Standby is on before you power up.
• When powering the amp up, have the Standby switch already on.. Switch the power on, wait a minute, and turn Standby off.
• When taking a short break, put the amp back on Standby.  
• When powering down, DON'T put the amp on Standby. Just turn it off.
• Now, if you just turn it on without using Standby, it'll be okay. Standby just warms the Tubes up a little slower, so they will last longer
• When plugging in/unplugging effects or switching speaker cabs, It's always best to turn the Standby switch on.”