Bias/recitifier question

[mike9]

I have a 1965 N/R Deluxe I got in trade. Nice amp, but it came with a 16 ohm Lead 80 and a 5Y3 rectifier tube. I've since replaced the speaker with a Weber alnico silver bell @ 8 ohm and ordered the required GZ34 rectifier.

Question is this - do I have to rebias the amp when I replace the rectifier tube? The GZ34 sounds different than the 5Y3 so I'm wondering if the tech that worked on this for him did anything in order to run the 5Y3 that I should look for? I'm going to rebias for some new NOS RCA 6V6's, but just curios about that GZ34.

[Structo]

Do you know which circuit it is?

Does your amp have a tremolo or vibrato?

The later AA1763 and AB763 which is probably what you have is fixed bias and has a bias pot.

The layout calls for a -35V bias.
Although it isn't spec'd what the wall supply voltage is supposed to be it is most likely spec'd for 115v.
So by using modern 120v wall voltage your heater and bias voltages may be a bit high.
Something to check out.

The choice of rectifier can change the B+ voltage since different tubes drop different voltages.
The 5y3 drops 60v from B+.
I can't find how much the GZ34 drops.

[Firestorm]

The blackface Deluxes and Deluxe Reverbs pushed the envelope on plate voltage on a 6V6GT: 415-420V with a wall voltage of 110VAC. Plug it into a 117-120VAC socket and you've got somewhere upwards of 450VDC on the plates, so the tech was probably trying to be nice to the tubes by replacing the GZ34. It will increase idle voltages by 20 volts or so. RCA 6V6s can take it, but you'll wear them out quicker. JJs and EH 6V6s can take it too, but don't sound as sweet.

Yes, you'll need to adjust the bias with a GZ34, but you may not be able to get the negative bias voltage high enough to compensate for modern wall voltage. In that case, you need to change the 10K bias load resistor on the bias pot to 15K or so.

Since this amp doesn't have reverb, it also doesn't have the reverb mix stage that makes the DRs the tonemonsters they were. And since it does have tremolo/vibrato, the loading effect of the circuit and the absence of that third gain stage really kills any chance of overdrive.

If you don't use the trem, disconnect it at the pot and you'll have a nice semi-tweed Deluxe. In that case, leave the 5Y3 (or try a 5V4).

This amp has the same transformers as the Deluxe Reverb, so it can handle a 6L6GC in the output. Again, you'll have to rebias, but then you can use the GZ34 easily, since the 6L6s will like the higher voltage.

Sorry to say, this is one of those amps where the choice of GZ34 will make a difference. Nothing will sound nearly as good a Mullard. Big $$$, unfortunately.

[mike9]

double post

[mike9]

I checked the label in the cab and it says: Power supply 117 volts 50/60 cycles AC. The circuit is AB763

Thank you for clearing that up - I'll try a 5V4 too just for the hell of it. It really didn't sound all that great with the GZ34 - that's why I was wondering. I'll pull the chassis and check the bias resistor. I like the sound of the RCA 6V6 black plates, but it sounds better with the 5Y3.

[Firestorm]

I guess the other caveat would be to check the speaker. These originally came with Oxford 12K5-6s that were underspecced to begin with: an 18-watt speaker in a 22-watt amp. So if you use a GZ34, or try 6L6 output tubes, you will kill that speaker if it's original. BTW, the Italian Jensen reissue C12N sounds killer in this amp.

[jjman]

Using the normal rectifier should provide a normal range of bias adjustment which would normally be more than adequate. My '71 (same values) provides a colossal amount of range. You could use a 5u4gb if you want to be “in between” the 5y3 and the stock GZ34. Re-bias after every change in Rectifier type since they all change the plate voltage.

My ’71 DR has almost the same strength from the Normal channel as the Vib channel and either is more than capable of driving the 6v6s into distortion. Looks like the preamp voltages are very similar between the BF non-Reverb, BFDR and my SFDR. I understand the Reverb channels have an extra gain stage in the preamp but it is set up with a voltage divider of a 3.3meg and a 220k (grid leak) which attenuates the signal going into that 3rd stage from the 2nd stage of the Rev channel. I feel this makes the Reverb channel’s final output only marginally louder than the adjacent Normal channel, especially since (as mentioned) the Trem pot also attenuates the output of that 3rd stage.

I’m thinking the Normal channel on a Stock BF Deluxe NR should be very close in preamp output to the Vib channel on a Stock DR.

[mike9]

I have no problem with gain on this. I'll try it with different rectifier tubes and set the bias accordingly. I know these RCA's can take a little extra heat, but I don't want to burn them up needlessly. I might go with either a MM, or a Heyboer Fat Stack OT though to beef up the low end.

[breakfastbuddy]

then of course you go down on the value its 10k now ang you go down so you can open for more negative voltage , if you go up to 15 k you get less negative voltage

[mike9]

It came with a 15 ohm Rola Lead 80. I tried a Weber alnico silver bell 50 watt'er, but I'm trying different speakers. FWIW one of the better sounding speakers I've tried is the Fender/Emi Legend 125 50 watt'er that's stock in the Hot Rod Deluxe. Sounds quite nice in a 22 watt amp. I have a C-Rex 16 ohm in it at the moment and that sounds nice as well. I ordered a Heyboer "Fat Stack" multi-tap OT from Weber today. That should beef up the lower end of this puppy. As long as it's not totally stock I don't mind souping it up a tad.

[Firestorm]

Running a 16-ohm speaker on an 8-ohm tap won't help your low end. It'll be interesting to see what the MM tranny does for it. The Vibrato channel has a 47pf bright cap across the volume pot that makes these a bit bright until they're cranked. If you want more bottom, try kicking the .001 PI coupling cap up somewhere between .002 and .005.

[doctord02]

For what it's worth, I've also got a '65 n/r Deluxe. Mine was modded by Dave Allen to have a Brown preamp/tone stack (volume/single tone knob) in the normal channel and an improved vibrato on the vibro channel - it has a switch on the intensity knob that removes it from the circuit. Very useable mods and the 6G6 (brown) channel is a *monster* .

I replaced the Weber speaker that was in it with an Emi Red Fang, and now it's a very cool useful amp. It was modded when I bought it, but I grabbed it because I like Brown Fenders and could not afford an actual Brown Deluxe. Dave Allen is generally one of the go-to guys for Brown circuits, so I knew it would rawk most mightily... And it does.

[mike9]

I just put the C-Rex in there to see what a hemp cone would sound like. I don't worry about the mismatch in a Fender. The Heyboer will have a 16 ohm tap so the impedance will match should I keep this speaker in there. It would be nice to have BF on one channel and Tweed on the other - Vibrato I could live without. I'm just going to do one thing at a time then listen to the amp and tweak and tune.