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

jellybelly

Hi All!

so I got a great deal on a DR Z EZG50, which was sold as having had a full set of tubes installed and serviced recently. However, when I got it home I found the ‘post’ volume to add quite a bit of noise if it got beyond 8 o’clock.

Then, on the third time of playing it blows a fuse. I follow the advice on Dr Zs website to track down the problem - power valves and rectifier are fine (although the rectifier is an old Marshall valve) but the fuse trips only when the preamp valves (which, again, are a hodge podge of old valves) are in and it’s flipped from ‘standby’ to ‘on’. This makes me think it’s a preamp valve at fault but I’m worried I could be damaging the amp if I replace the fuse and keep trying - I’m hoping since the rec and power valves are ok there’s not much I can do to hurt it but I don’t want to test that theory on an expensive bit of gear!!

Also considering just getting a new set of preamp valves, which should solve the problem if it is a valve issue and not something more serious. Is it worth confronting the seller regarding it not having a set of new valves? It’s not like the stuff in there are NOS or anything - old Fender, Sovtek and Tung Sol RIs...

Anyway, advice greatly welcomed!!

Modulus_Amps

If you pull the valves and it does not blow a few then change the valves one at a time to find the faulty valve.

I have never had a pre amp valve fail to cause fuses to blow, others can chime in on that, did it get couriered to you?

either way I would avoid having the HT fuse blow too often when you don't know why it is blowing, it may be arcing internally which will cause further issues/noise.

Did he say it had new valves installed, your text form above said "had a full set of tubes installed and serviced recently" Maybe he has a copy of the service report from the tech who did the work

ICBM

It's very unlikely to be a preamp valve - they normally can't draw enough current to blow a fuse, even in a cathode-follower position (which I have no idea if that amp uses) because there are large resistors in the power supply chain.

My guess is the rectifier valve.

Because: with no preamp valves in, there is no current draw from the preamp and the voltages on the filter caps will rise to the full HT. (By the way, with vintage amps it's a bad idea to do this, as it can exceed the cap voltage rating - *most* modern amps aren't as much at risk.) This means that there will be very little additional current draw when the standby is turned on.

But: with the preamp valves in, the filter caps will drain slowly through the valves when the amp is on standby. When you flip the switch, there will then be a fairly large current surge as they charge up - when empty, they're effectively a short on the supply for a fraction of a second until the voltage rises. This could be making the rectifier valve arc, which will then blow the fuse.

This is a common problem with many modern amps with valve rectifiers and standby switches, which are often in the wrong place in the circuit so there is always a big surge in the rectifier whether or not the preamp valves are in. (Orange and Vox among others.) I don't know if this amp has the same arrangement.

As Modulus_Amps said, try to avoid using a blowing fuse as a troubleshooting mechanism if you can - it's not good for the rest of the amp really.

jellybelly

Thanks both for your comments - my gut was saying preamp valves don’t ‘do’ enough to cause that sort of issue, at least not instantly. I have seen a note by Dr Z on his forum recommending ‘slo blo’ fuses - @icbm would that stop the initial surge when the standby switch is flipped?

I might try once more after replacing all the valves including the recitifier (the power valves do appear to be new so will leave those) and after that wait until a tech can take a look. 

jellybelly

@Modulus_Amps yes he said a full set. I should have pressed him in brand etc but frankly was too keen to get it out the door for the price I paid!

Modulus_Amps

One thing I would do is to stop using the standby switch, just leave it in the on position, the rectifier valve heat up slowly giving a soft start and this will avoid any inrush currents like ICBM mentions and will be easier on the fuse...

Definitely do not fit a bigger fuse or one rated differently or you may cause allot of damage to the amp

ICBM

jellybelly said:

I have seen a note by Dr Z on his forum recommending ‘slo blo’ fuses - @icbm would that stop the initial surge when the standby switch is flipped?

If he says so then I would use a slow-blow (T) fuse, yes. It won't stop the surge but it may be just that which is blowing the fuse.

Was the amp shipped to you, and were the valves left in? Shock damage to power valves and rectifiers is quite common if the amp is dropped. It won't be the power valves if it only blows the fuse with the preamps in though.

What actual manufacturer is the Marshall-labelled rectifier? If it's a Sovtek I would suspect that even more.

jellybelly

Collected the amp locally, though as I say there it was noisy with the master turned up which made me suspect something was up. I’ll get a new rectifier and preamp set. 

Would you guys recommend JJ or Tung Sol for rectifiers and 12AT7s? I already know I prefer Tung sol 12AX7s and JJ power valves but not much experience with the rest...

ICBM

JJ for the rectifier definitely.

I prefer Tung-Sol preamp valves too, I don't like JJs.

jellybelly

ICBM said:

JJ for the rectifier definitely.

I prefer Tung-Sol preamp valves too, I don't like JJs.

Yeah, assuming the 12at7s have similar differences I’ll definitely go for Tung sols. Cheers for your help guys. 

peteri

One thing to add - I had a Dr Z Mini Z for 18 months or so, really enjoyed it - although eventually tired of it's quite brutally exposing/treble tone.

One thing that I didn't like was it seemed to get through a lot of valves, as a home player I've never had to replace a valve, got through three in a year on that bad boy. From memory all preamp

Not sure if that means anything!

ecc83

Are they perhaps running the heaters a bit hotter than 6.3V?  One of the incidental benefits of regulated DC heating is that pre amp valves seem to last forever!

This was also my experience with 'old' valved hi fi and PA amps where the  traffs were wound with perhaps greater precision? In any case I think mains voltage was a bit more stable and clean?

Dave.

hywelg

peteri said:

One thing to add - I had a Dr Z Mini Z for 18 months or so, really enjoyed it - although eventually tired of it's quite brutally exposing/treble tone.

One thing that I didn't like was it seemed to get through a lot of valves, as a home player I've never had to replace a valve, got through three in a year on that bad boy. From memory all preamp

Not sure if that means anything!

No not really. I had an EZG for quite a while, changed a pair of worn out (when I got it) power valves for a pair of Winged C's and they lasted all the while I had it pretty much. Don't think I ever change a pre.

There was an issue with some early EZG's with the grounding of the reverb transformer. DrZ was excellent and sent me a full set of labeled photos showing how to cure the issue. That problem was also manifest when turning the post volume up.

If you (OP) want a copy of the pre-amp schematic (Dr Z dont give out amp schematics which is mad) that i drew please PM me.

ICBM

peteri said:
One thing to add - I had a Dr Z Mini Z for 18 months or so, really enjoyed it - although eventually tired of it's quite brutally exposing/treble tone.

One thing that I didn't like was it seemed to get through a lot of valves, as a home player I've never had to replace a valve, got through three in a year on that bad boy. From memory all preamp

ecc83 said:

Are they perhaps running the heaters a bit hotter than 6.3V?

The one I worked on was (badly) re-purposed from a larger amp chassis designed for more valves. I didn't check the PT spec, but there could well have been more than 6.3V on the filaments.

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

koss59

I was interested in buying one of these once and when doing some searches on here I found a thread with someone having this same problem I think. I wonder if it’s the same amp and has never truly been fixed.......

Modulus_Amps

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

jellybelly

@hywelg  have pm’d you mate, help is much appreciated!

@koss59 any chance you’ve got a link to that thread?? Thought I’d researched it pretty well on here. Am commited to getting it sorted - it should be doable, I can’t imagine a well-made amp NOT being fixable (assuming this is a bit better than the one ICBM posted about!!). Got the amp for a steal of a price, which perhaps should ring alarm bells but ultimately I’m willing to put a little money into it to sort it out...

Wazmeister

koss59 said:

I was interested in buying one of these once and when doing some searches on here I found a thread with someone having this same problem I think. I wonder if it’s the same amp and has never truly been fixed.......

There have been a couple like this it appears...

A great amp though, and Id definitely own another.

ecc83

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.

Modulus_Amps

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.

koss59

Here you go

http://www.thefretboard.co.uk/discussion/91996/