Blown PT?

[WA4SWJ]

Some time back I posted that I lost a JJ GZ-34 in mt ST-70 clone to arcing. It arced over and there was no problem with the amp. I replaced the tube with a solid state rectifier replacement board with even more capacitance and have had no more problems. I have read various reports that even the Mullard 5AR4/GZ-34's are not all that great either. I think the basic problem is that they are right at their limit in the ST-70 amp. I'm sure they are fine when run conservatively. I have some Sovtek 5AR4's that I am using in my Mark III/Poseidon monoblock clones. We'll see how they work out. So far no problems.

Hope all works out well.

Regards,

[EWBrown]

Cake pans are good for "slapped together" prototypes and real cheapo amp stuff, nothing better than a $1.39 chassis, except for perhaps a free one (AKA dumpster chassis) and I specialize in those, too. If you think that Hammond chassis can be flimsy, y'ain't seen nuthin' yet Yellow_Light_Colorz_PDT_07

FWIW, there's a C354 choke and another 'lytic, and a couple of resistors hiding inside.

That little power supply on a cake pan buzzed like a cheap electric razor, from the vertical mount trannie's stray magnetic field. I had to use some strips of black rubber and double sided foam tape to get it down to a dull roar.

(Hint: there's a reason why God invented aluminum chassis) Yellow_Light_Colorz_PDT_16

Heck, I've even done the 1920s "retro" thing and just used a wooden plank (breadboard) and screwed every thing down and wired it all point to point. It works, but it isn't always pretty. For octal tube sockets I use the "relay sockets" that have screw terminals.

/ed B in NH

[craigtone]

I just won a pair of NOS Military Mullard GZ34's for $78! Yellow_Light_Colorz_PDT_02

Due to the scarcity of these puppies, what can I do to help protect the rectifier? Switching to solid state is 100% out of the question! No offense to anyone, just not my bag! :)

I have read a few threads about the current limiters ... anyone running one of these? What are the pros/cons of these?

Even though I'll have a spare, I'd like to preserve these as long as possible.

[TomMcNally]

Just watch the value of the first capacitor off the rectifier tube - it should be smallish. The ancient Mercury Vapor # 83 I use in my 300B amp only wants to see about 3 mfd ! After the choke, you can go with higher values. 20 to 40 mfd is probably safe for a GZ-34, but that cap is like a brick wall when it's cold ! smack !

I put GZ-37's, the big brother to the GZ-34 in my new Poseidon/MK-III clones. I haven't measured B+ yet, hate to disconnect them, they sound sooooo good !

I should put some of those CHIAN Election specials in and see a show !

[craigtone]

Good idea! I have the 80-40-30-20 wired as 40-80+30-20 right now. I could rewire it to 20-80+40-30 ...

[EWBrown]

That, or try 30-L-80-R-30-R-20 might be a good compromise. I usually limit it to 33 uF at the tube rect cathode. Those "cap board" filters can cut it close to the limit of the tube's tolerance.

SS rect are more forgiving, in that they can feed a high value electrolytic, thogh using an ICL in the primary is a real good idea, (as in the DIY35 or Ike boards). I've gone straight into 470 uF and even two of them in parallel for 940 uF with no ill effects. Try that with any tube rectifier and it will detonate Yellow_Light_Colorz_PDT_09

The other trick with MV rectifiers like the 83, is to use a hefty wattage 10K resistor in series with the input cap, and use a time delay to short it out after the MV rect has had time to properly warm up. Maybe do this from CT to ground, at least the time delay contacts aren't always at max B+ voltage.

/ed B in NH

[craigtone]

That, or try 30-L-80-R-30-R-20 might be a good compromise. I usually limit it to 33 uF at the tube rect cathode. Those "cap board" filters can cut it close to the limit of the tube's tolerance.

I have the VTA driver board, so right now I am running 40-L-80+30-R-20. His board has it's own B+ RC filtering for the preamp, thus the jumpering of the 2 stages of the quad cap. Normally he suggests jumping all 3, but I have a 2.2K between the last 2 stages to reduce B+ because of the higher line voltages than the amp was originally designed to see.

I might do 30-L-80+40-R-20 instead ...

[TomMcNally]

I have an Amperite time delay tube that locks up a 6.3 VAC coil relay and then releases itself - as suggested by Amperite in an old doc I found. I'm opening the CT from ground directly - and it does make a nasty POP when it engages - the 10K resistor sounds like a better idea. Now just come down here and help me turn this beast upside down ! haha

[mesherm]

Craig,
On the subject of current limiters.
I wire two of the CL90 current limiters in parallel and then wire that combo in series in the PT primary of all my ST70 clones and it works well. I usually bolt a vertical phenolic solder lug tree to the mounting bolt for the choke closest to the PT and make sure there is clearance for heat dissipation.
Mike

[craigtone]

Thanks for everyone's replies! I re-wired it and the problem is gone. I'm going to attribute it to a sloppy wiring job (since I knew I was re-wiring anyway) and a Fast-Acting Fuse (vs. a Slo-Blo).

The amp is refurbished and sounding better than ever! See my other post for pics and results.