It is currently Mon Mar 16, 2020 9:04 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 18 posts ]  Go to page Previous  1, 2
Go to page Previous  1, 2
Author Message
Post subject: Re: Tube ID
Posted: Wed Aug 30, 2017 7:46 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
This is an old Dynaco mod. Their ST-70 uses a single GZ34/5AR4 is power four EL34 (35 watts per channel stereo). When ppl started putting Russian and Chinese made "GZ34" into their amps, in the 1970-80s --- bad things started happening. Like catastrophic shorts that took out trannies, before the fuse could pop.

The solid-state diodes are inline, before the tube rectifier. Here's a fairly good explanation of how the two diodes work. BTW... the mod is to use UF4007 diodes, these days. The way the diodes are wired into the circuit, you maintain the slow-start of the GZ34 and the tube sag --- under normal operating conditions.

"One SS diode in series with each anode of a vacuum rectifier protects the power transformer, filter caps, probably the choke from a shorted rectifier tube. A rectifier fuse ices the cake. With the diodes in place, your rectifier tube shorting, causes a change in tone, but you finish the gig, and you may not even notice. Without them, the amp dies. Some amps have caught fire from shorted rectifiers.

Will a line fuse protect the amp from a shorted rectifier? Maybe. Maybe not. I've seen it both ways. Fuses cost about $0.25. 1N4007's are 0.04 each. I'd spend $0.16 to not ever put my AC line fuse's protection qualities to the test.

Naturally, if there is no heater supply to the vacuum rectifier or if the rectifier tube has a burned out heater filament (indirect), there is no amp operation because the rectifier tube does not conduct. But the failure is contained to the rectifier tube.

Does that explanation help? Please correct me if I'm missing something.

I recommend 1200V / 1A or more FREDs for the diodes. They're under $5 each. You can use one or more 1N4007s for them, but 1KV is tight on voltage rating, and you need to parallel them with 1M's to equalize reverse leakage if you series them."


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Tube ID
Posted: Wed Aug 30, 2017 2:20 pm
Offline
Professional Musician
Professional Musician

Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
Thanks for the info Beemer...
I guess when I run out of tubes, a SS rectifier will be the ticket.
I'll add a couple dozen uf4007 to my next mouser order.


Top
Profile
Post subject: Re: Tube ID
Posted: Wed Aug 30, 2017 6:49 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Image

If you look at the diagram, you can kinda tell how this mod works. As long as the tube rectifier is okay... its character dominates the tone. But, if the tube's anode plates short to the cathode ---- the solid-state diodes prevent pure secondary AC PIV from entering the power supply. Which is very bad for filter caps and things downstream.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 18 posts ]  Go to page Previous  1, 2
Go to page Previous  1, 2

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: