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DR Blows Reverb Driver Tubes

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12/7/1999 11:17 PM
jc	DR Blows Reverb Driver Tubes I just bought an SFDR. I replaced the filter caps. Clipped the tonesuck caps. Installed NOS tubes and rebiased. The thing is it keeps blowing reverb driver tubes. I've gone through about 3. They get real microphonic first and then they stop working. Any ideas? Thanks, JC

12/8/1999 12:48 AM

Is it a pull boost model?

The cathode resistor on the reverb driver is possibly a low value, and that's very hard on the driver tube. Check it. If you find a 470, 680 or 1k resistor there, up it to at least a 1.5k (1W). The black face falue is 2200 ohms(1/2W).

I'd seriously think about disconnecting the pull boost (if ya have it) and maybe doing the full blackface mods.

Other than that, I've heard that the Philips JAN 12AT7 will survive in that position for a while with the low 'stock' SF values.

(information here courtesy of Willie Whittaker)

HTH! - Andrew

12/8/1999 12:52 AM
Andrew	I should have also suggested that you check the cathode bypass capacitor! It may be shorted. Suggest 25湩/50V to replace it, if it's shot.

12/8/1999 1:08 AM
jc	Thanks Andrew. I'll check that tonight. I think that it does have a 680 resistor there. I have disconnected the pull boost. What else needs to be done to blackface this baby? jc

12/8/1999 3:03 AM

SF to BF conversion/Deluxe reverb

Comparing this:
http://www.geocities.com/Area51/Nova/4551/schems/deluxe_reverb_10-6-77_schem.gif

with this:
http://homepages.tig.com.au/~mcwhirt/andrew/images/dr_ab763_schem.gif

These are the differences (note that I wouldn't advise 'undoing' those marked with a *, either because I dunno what they were for, or it won't make any difference in sound, or it really was an improvement!):

SF BF
===================================
Phase splitter input:
330k 1M
?can't read? 470

Caps from grid-ground
1200pF none fitted

Power supply first dropper resistor:
2.2k/1W 10k/1W

*Bias supply cap
*80湩/75V 25湩/50V

*Fuse
*2A sloblo 1A sloblo

*CSA Thermal protector
*fitted none fitted

Vibrato second stage bypass cap (hard to read on schematic?)
?5湩/50V? 25湩/25V

*6.3V Filament supply
*2 x 100 ohms to gnd gnded C/T

Pull boost mixer resistor
1k none
(220k connected to gnd)

Pull boost switch
fitted not fitted

reverb recovery grid bypass(?)
2000pF none

reverb driver cathode
680ohm 2.2k

reverb driver cathode bypass cap
none 25湩/25V

*recifier tube
*5U4GB GZ34
(you could try a GZ34, no harm done. Check your B+ voltage first though.)

That's all I can see. Hope it helps ya - Andrew

12/8/1999 5:20 PM
Gerben	You can also find some general info over here: http://www.sonic-data.com/~andy/mga/main/bf_vs_sf.htm Gerben

12/8/1999 7:58 AM

Re: DR Blows Reverb Driver Tubes

JC
The tone suck caps ,as you may have read, were added to eleminate oscilation from the lead dress problems. This high frequency oscilation can cause tube ringing, crack the glass, and lower output power. sometimes you can't hear it but you know your having these problems. I had this problem in this same amp. Replace the input lead and the volume control lead with shielded cable and ground only one end. Usually the source end or the one furthest from ac noise. This really helped in my amp. I would go with changing that 680 cathode resistor also. If you have a 5u4b recto don't change to the gz34. The voltage goes up too high on the plates. Also the amp doesn't sound the same with the higher preamp voltages. hope this helps.

Tracy

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