Fender Dlx Rev build- last Q b4 fire-up.

[Sea Chief]

Hi ChuckH,

As I mentioned from the off Chuck, Im not using a DR power xfmr (tried and tried but couldnt find an export one over here/ fiercely costly in total to have one if found said lump, post etc). Im using a hammond 291BEX and a GZ34S rectifier. Mojotone OT. 330-0-330 its meant to be (but if I read 700v before I flipped STBY then thats actually 2x 350v right? so is that a prob one, 'bad of a batch' one d'ya think?

Anyway it lives- so thank goodness for that anyway. Signing off/ its early AM hrs now. Cheers SC

[Chuck H]

The unloaded primary for that PT is 701.89V. That would be close enough to what you're reading with the standby off. So...

If you change to a 5U4 rectifier and bump the current a little you should get about 428V with the tubes biased at around 70%. This according to Duncan PSU which, when programmed right, hasn't steered me wrong.

That would be more typical of what DR's do with modern plate voltages here on the other side of the pond. The 5U4 is a SF incarnation so it's possible that Hammond garnered specs from something other than an actual BF amp. No big deal IMO. Get a 5U4 and all should be closer to what one would expect. OTOH...

There have been a few people here that have run what seems like silly high voltages on 6V6 tubes. I wouldn't. I've blown up a few trying and had my fill. Whatever you do, I don't recommend clipping the power tubes at that voltage. Instant death IME.

[Sea Chief]

Instant death..? well I hadn't got that on my list of things-to-do today defo.

A 5U4? (Is that a 5U4G or GB same?) so thats another £20 to spend.. hang on if a Co (Bluebell audio, very personable on phone and did mention +/- 20% tolerance on the HV pair) sells xfmrs as 'for Dlx Rev' should I really be looking to have to spend another £20 cos the v are 95 ott spec to a modern Dlx Rev?? seems to me s'thing amiss there.

[Chuck H]

Amiss? Absolutely. You can be upset about it if it helps. Don't shoot the messenger. The trouble is that the specs are given. It's up the designer/builder to handle it from there. The BF DR schematic shows 330-0-330 PT, a GZ34 rectifier and 415Vp. The SF DR schematic shows 330-0-330 PT, a 5U4GB rectifier and 415Vp. Wait,.. What? A GZ34 has a Vdrop of 15ish and a 5U4GB is closer to 50! How can both schematics be accurate?.. They aren't. Call Leo and complain

I didn't have the PT specs until post #31. Once I had them I used Duncan PSU to determine the approx. Vp with that PT and found that a 5U4GB tube would be needed in post #32. I had the specs and worked it out prior to buying a GZ34 rectifier. Because you didn't, or can't, or the amp just came with that tube, or whatever, you now have to spend another twenty bones. Don't shoot the messenger.

As to the instant death and whether the amp is actually ok with a pair of 6v6's at 478Vp. Plowing ahead with the caveat "They (Hammond) said it's ok, so there you go." Well... You can believe me now or possibly buy a 5U4GB and another pair of 6V6's later if you ever intend to clip the power tubes at all.

[guitician]

-35V DC is what I see shown as the bias for the DR and 420V DC on the plates. Drawing current from the power supply will bring down the voltage.

[Sea Chief]

Chuck,

sorry Im not quite following the thread of your post. Are you saying its my fault, just get a 5U4GB.. or you should have got a 5U4GB with the PT not a GZ34S as I knew it was a 330-0-330 xfmr-?

I did mention #1 the Gz34S. And are you suggesting the power tubes, at the V I read, are unuseable and 'instant death' is a possibility for them running at this voltage (anytime, or after a cut-short but normal timeframe?) Or is your implication instant death to the amp xfmr I assume? (or me?!)

[Sea Chief]

-35V DC is what I see shown as the bias for the DR and 420V DC on the plates. Drawing current from the power supply will bring down the voltage.

Im sure it might be true.. but I dont understand, apologies.

[guitician]

Right now, with -50V dc bias, the tubes aren't loading the power supply thus lowering the B+ from the 478V to maybe 430V. No way to tell until you set the bias down to a less negative value and watch the B+ voltage as you lower the bias voltage and you will see the 478V drop. Cheers

[Sea Chief]

Right now, with -50V dc bias, the tubes aren't loading the power supply thus lowering the B+ from the 478V to maybe 430V. No way to tell until you set the bias down to a less negative value and watch the B+ voltage as you lower the bias voltage and you will see the 478V drop. Cheers

Ok understand the basics of that, thaanks.

So forgive my ignorance but whats to say you could just turn down the bias pot to acheive a less negative value-?

[eschertron]

but whats to say you could just turn down the bias pot to acheive a less negative value-?

Exactly. the -50vdc (I'm assuming the bias pot is turned all the way one way, to the 'coldest' bias setting) keeps the 6V6 tubes from conducting hardly at all. As you turn the bias pot, the bias goes less negative or 'hotter' and the tubes conduct more at idle. Too much and the tubes melt down but just enough and the extra current drawn at idle will bring the B+ voltage down to a more sane level. In theory. This is where a bias probe comes in handy so you know how much is too much. Before it happens!

Be sure to remember there are real limits to the stress that these tubes can handle before Chuck's 'sudden death' scenario plays out. If you can, get your hands on the recto that drops more voltage. Even if you don't use it in this build, it's another tool in your arsenal.

[Sea Chief]

Ok understanding more of this bias circuit eschertron. Good.

The pot I set at pretty exactly middle position tho, and -not- dialled one way. So id better not touch it I guess in case in case I push it 'up'. And I dont know which way is up/down on the pot: and can only turn it from underside too.

So I need to know, looking at the underside of amp, up and down on the pot.. assumption twds RHS is up but no assumings.

Im still not getting why I cant measure the mA on each power tube, and dial the pot to the desired mA of "25mA" let's say (I have a feeling Ill be chastised for saying this again).

[Tom Phillips]

...I'm still not getting why I cant measure the mA on each power tube, and dial the pot to the desired mA of "25mA" let's say (I have a feeling Ill be chastised for saying this again).

You can and you should monitor the current through each tube as you adjust the bias voltage. I recommend that you add 1 Ohm current sense resistors between pin 8 and ground of each 6V6. Reference the discussion staring in post #8 of this thread. Also Google for lots of available explanation of this bias current monitoring technique.

[Chuck H]

I'm not saying the selection of a GZ34 rectifier is your fault. Info to use that tube with a 330-0-330 PT is right there in the schematic! (incorrectly) It's unfortunate to be sure. I'm only saying that it wouldn't have happened to me. And probably not most electronically literate builders. Not that I'm that damn literate. Experience and knowledge would have helped you skirt buying another rectifier tube. But that didn't happen and now your best move would be to buy another rectifier tube.

Increasing current will bring voltage down. But IME it's not going to bring it down enough. At some point it's diminishing returns. You can't just keep increasing current until the voltage is more ideal because the tube will melt down. So, with the setup you're using, the worst case scenario would be that the tubes will idle fairly hot and the plate voltage will still be a good bit too high. Does that seem like a good idea? Because it's your call ultimately.

I can't tell, but it seems like you're getting grumpy with me for some reason. If you are, I'd like to know why.

[g1]

Im still not getting why I cant measure the mA on each power tube, and dial the pot to the desired mA of "25mA" let's say (I have a feeling Ill be chastised for saying this again).

Measuring current with a meter requires breaking the circuit so you can insert the meter in series. You would break the connection between pin8 and ground. Once you have done that, you may as well add the 1ohm resisters in there. Then next time you don't have to break the circuit, you just measure voltage across the resistor and calculate the current using ohms law.

[Chuck H]

Measuring current with a meter requires breaking the circuit so you can insert the meter in series. You would break the connection between pin8 and ground. Once you have done that, you may as well add the 1ohm resisters in there. Then next time you don't have to break the circuit, you just measure voltage across the resistor and calculate the current using ohms law.

g1! You had a nifty interjection earlier in this thread suggesting that the resistance of the OT (presumably from CT to an end) could be used to calculate plate current. This seems like a nifty idea to me! Not sure what the drawback would be. Impedance and all might be more important for conduction, but for idle current it seems like it should work fine.