Metropoulos Forum

Hi all, I did a quick search on this topic & found posts explaining it but nothing saying whether or not its reccomended. So wot do u guys think, on a JTM50 clone using EL34's (JTM45 PT), is the use of the diodes & cap in conjuction with the GZ34 reccomended or not, & does it affect the GZ34's characteristics etc. Any comments appreciated. Cheers

The diodes are intended to provide some level of protection if the GZ34 melts down. For instance if the GZ34 shorted plates to cathode the diodes would ensure the polarized filter caps do not see large swings of negative voltage with potentially violent, smoky consequences. The cap is used to squelch diode switching related noise. I have them installed on my 18W TMB and really do not notice any negative affect. Scott

Edit: For your understanding the Ceriatone method is illustrated below. Look for the diodes and cap on the GZ34. But after some thought I like the single diode method described farther down. The single diode on the GZ34 output would always be conducting and eliminates possible diode switching hash from a pair on the GZ34 input. I am about to start a KT88 based tube amp kit (stereo audio) and plan on trying the single diode method. And after thinking about it feel the diodes could be very important in reducing potential AC power interruption short cycle reverse current arcing in the GZ34. This is perhaps the best reason for inclusion in the circuit.
http://www.ceriatone.com/images/layoutP ... iatone.jpg

Thanx Scott. Kevin O'connor, designer, London Power amps, author TUT series etc has advised against this mod, & against putting a cap (the 0.01uf/3Kv disk) across 2 diodes, recommending instead 2 x (around) 47 ohm/5W resistors between PT & rec plates to limit the current surge of the filter at switch on, which is considerable & strains the rec tube, & suggests a single diode as the DC path off pin 8 to protect against AC in the event of a rec tube failing. Cheers

Oh, he also said that diode switching hash is not an issue in guitar amps. See DIY Audio forum where I also asked about this mod in the Instruments & Amps section for his input on subject. A couple of other seemingly knowledgeable people also chimed in. Cheers

Excellent suggestions and worthy of consideration....

Interesting.

Do you have a link so that I may have a read?

I have looked through a few pages of the forum but did not see the thread.

ivan H wrote:Oh, he also said that diode switching hash is not an issue in guitar amps.

If this were true, we wouldn't notice any difference when upgrading IN4007's to UF4007's or UF5408's. I would have to dispute this claim.

Although what he actually said was it is not of concern, which is a little different. It is there, just limited. But certainly can improve many amps when reduced.

neikeel wrote:Interesting.

Do you have a link so that I may have a read?

I have looked through a few pages of the forum but did not see the thread.

Pretty sure this is the thread in question http://www.diyaudio.com/forums/instrume ... f-big.html

Oh, he also said that diode switching hash is not an issue in guitar amps.

Power supply hash/hum drives me crazy. That is my next project on the JMP50 - reduce hash.