quick dropping resistor question

[alchemy]

Assuming that the resistor is of sufficient wattage, is there any reason not to use a dropping resistor right after the rectifier diodes to drop the voltage going to the OT center tap and plates of output tubes? I went through a lot of schematics and I can not find any examples of amps that do this, so I wanted to be sure. Thanks and cheers!

[Wilder Amplification]

Assuming that the resistor is of sufficient wattage, is there any reason not to use a dropping resistor right after the rectifier diodes to drop the voltage going to the OT center tap and plates of output tubes? I went through a lot of schematics and I can not find any examples of amps that do this, so I wanted to be sure. Thanks and cheers!

There are lots of people who use a resistor there to simulate a rectifier tube and add sag. As long as its of a sufficient wattage rating and your intent was to introduce sag it would be fine.

However if you're just wanting to drop plate voltage it would be better to use a zener diode in either the PT center tap or the (-) side of the bridge rectifier, depending on which rectifier setup you have.

[GibsonLover]

You can position the sag resistor on the PT center tap lead too, in place of the proposed zener. One could even use both, depending on how much B+ reduction you are looking for, and still have a little sag (if sag appeals to you). I wouldn't use more than 100 - 120 ohms for the sag resistor in any position. Figure your B+ current, and the resistor value, and decide if the voltage drop is enough before employing a zener.

[alchemy]

I am not familiar with using zeners or resistors on the CT to drop B+. Any enlightenment or pointing me in the direction of info would be great.

I am trying to drop 50-60v. It's in an old Panaramic that someone hacked up. The replacement PT is rated 310-0-310 but is giving me 450 or so on the tubes.

I have been playing around with resistor values between the rectifier and the node for the OT center tap, and it looks like i'm gonna end up with about 200 ohms to get the B+ down where I want it.

I'm not neccesarily opposed to sag, but I'd like to try it both ways and see which sounds better.

[Wilder Amplification]

I am not familiar with using zeners or resistors on the CT to drop B+. Any enlightenment or pointing me in the direction of info would be great.

I am trying to drop 50-60v. It's in an old Panaramic that someone hacked up. The replacement PT is rated 310-0-310 but is giving me 450 or so on the tubes.

I have been playing around with resistor values between the rectifier and the node for the OT center tap, and it looks like i'm gonna end up with about 200 ohms to get the B+ down where I want it.

I'm not neccesarily opposed to sag, but I'd like to try it both ways and see which sounds better.

OK BEFORE we go any further, a few things we need to know first -

Is that 450V with or without the tubes in?

The PT may be rated 310-0-310, but what is your wall voltage and what is the ACTUAL AC voltage you're measuring on that HT secondary?

What is your LOADED B+ (i.e. OT center tap voltage with tubes in and running in "play mode") and what is the voltage you're trying to get to?

[alchemy]

Wall voltage is 121-123v AC pretty steady.

450 with the tubes in.

The HT voltage is too high for my meter, well over 600v AC. From one leg to CT it's 446v AC

This amp originally had a bridge rectifier (and no center tap on original PT), but since the replacement PT that was in it when I got it had a center tap, I built a full wave rectifier in it.

The loaded B+ is (right now, biased at 35ma) 442v DC. It has fluctuated a bit depending (obviously) on bias setting and the set of tubes in it.

[Wilder Amplification]

Wall voltage is 121-123v AC pretty steady.

450 with the tubes in.

The HT voltage is too high for my meter, well over 600v AC. From one leg to CT it's 446v AC

This amp originally had a bridge rectifier (and no center tap on original PT), but since the replacement PT that was in it when I got it had a center tap, I built a full wave rectifier in it.

The loaded B+ is (right now, biased at 35ma) 442v DC. It has fluctuated a bit depending (obviously) on bias setting and the set of tubes in it.

That doesn't seem right for your one leg to CT measurement.

In order for you to have 442VDC loaded, you'd have to have roughly 470VDC unloaded, and for that to happen you would have to be measuring about 330VAC from one leg to CT (330VAC x 1.414 = 470VDC...with tubes in that should load it about 20-30 volts).

Now the question is...why do you need a lower B+? What kind of power tubes are in this amp and how many? Single ended or push pull?

[alchemy]

2x7591 (JJ, not real ones), push pull. I want to drop the B+ because running the plates (and screens ) at around 450v just seems to be asking for it from what I've heard about the new production 7591. Also, since no schematic seems to exist for this amp, I have been roughly going off of a schematic for an older Versatone 7591 based amp, only because the power supply portion looked to be very similar (the preamp, etc. is not the same, only the power supply). The voltages listed on this schematic show approx 380 on the plates.
Not sure if it will work if you are not a member, but here's the schematic:
http://f1.grp.yahoofs.com/v1/8HkNTJC...rsatonejpg.jpg
The amp I have is a Panaramic 2x7591, 2x12ax7, 1x6gh8b. Single 12 inch speaker with reverb and tremolo. It is the same amp as the Audio Guild Grand Prix, as well as several others made by A.G. and sold under names like Titano, Universal, etc. No luck in six months looking for a schematic.
I have a guy at the magnatone website that is going to measure voltages in his and get back to me, but even these could be way off in a 40 year old amp. Just trying to get it up and running in as close to original condition as I can (minus all the power supply changes of course, which I understand will change the sound considerably.)

[Chuck H]

I get 421Vp using Duncan PSU. It's usually very accurate, but the age of the amp could have something to do with it. What is your PT primary input voltage (not your wall voltage, but the voltage the primary is designed for?

That aside, You said it's biased at 35mA. It that per tube?

A zener on the CT will drop volts without sag. A resistor in the CT or after the diode rectifier will rdop volts and create sag. It really depends on the type of sound your after. If you want to simulate a rectifier tube like a jtm45 Marshall or any Fender then a 200 ohm resistor seems fine to me. A 5y3 or 5u4 will drop almost as many volts as that resistor and this is due to the tube rectifiers internal resistance. That resistor will affect the power supply in a very similar manor. If you want a tight, plexi/Super Lead Marshall kind of tone then a zener in the PT CT will drop some volts and still keep a fast, hard power supply that you need for this kind of sound.

One thing that puzzles me is that you said the amp had a bridge rectifier arrangement when you got it. With that PT and a normal load it would have produced about 890Vp!!!

Chuck

[alchemy]

I'm not sure what the Duncan PSU is...I'd love to know...

PT is 120v primary.

35ma per tube, through 1ohm 1% resistor on cathode to ground. This was just where I had it set when I took the last measurement. I've been playing with it some.

As for the zeners, I'm going to take a guess that if I was to put one in the center tap, I would get a zener that had a voltage rating of the amount I wanted to drop. Am I way off? I said it was just a guess!

Is there an audible difference in sound between a bridge rectifier and a full wave?

Does anyone know anything about Allen transformers?

The amp did have a bridge rectifier when I got it, and yes it was putting out over 800 volts. I picked it up for next to nothing. The guy at the shop who had it said the output tubes had blown, but that it used to produce sound. As soon as I opened it up, I knew this was not true, but I didn't care as it was almost free and it has taught me a lot along the way. Someone had really hacked it up inside. It appeared that they had tried to replace both the PT and OT, but had given up somewhere along the way. Many things wired wrong/not wired at all. Center tapped PT with a bridge rectifier, but a splice in the line sending part of it to an unfinished full wave. Red plating plates and red hot screens. Etc etc etc....You get the idea. I ripped out the board that holds the bias supply and all dropping resistors and built a new one, replaced all filter caps (all blown), and slowly managed to get it working without any idea what I was doing at that point and no schematic to help. It's been fun, and taught me more about tube amps than all the other projects combined!

Thanks alot for the replies. Very helpful, as usual. When I get the voltage measurements from the guy who has the same amp, I'll tweak it to those specs and go from there.

[tubeswell]

As for the zeners, I'm going to take a guess that if I was to put one in the center tap, I would get a zener that had a voltage rating of the amount I wanted to drop. Am I way off? I said it was just a guess!

All you really need to know to work out the appropriate zener is the amount of voltage drop your want on your secondary High Tension winding and the overall HT winding current-draw rating. Say the HT is 310-0-310 @ 150mA, then 150mA is what you use (i.e.; the maximum current draw you would want through the HT winding anyhow). If you want a 24V drop, then using (say) 2 x 12V zeners in series, 24V x 150mA = 3.6W. Doubling that is 7.2W, so if the zeners were each 5W axial zeners, that's 10W in total, which is ample. If you are using 5W axial mount ones, I'd mount them on a tag strip off the chassis - that way they will stay nice'n'cool. If you are using a 24V bolt-on type zener, then get at least 25 W rating because of the heat it will pick up from the chassis. Zeners go cathode (banded end) pointing to ground in series with the centre tap of the HT winding.

[Wilder Amplification]

I was unable to pull up the scheme. Says "Document Not Found". Any way you can download it and post it to photobucket?

[Wilder Amplification]

I cant get photobucket to make it big enough to see. I'll trying adding it as an attachment. Keep in mind this is not my amp. This is an older Audio Guild that has a power supply and filtering almost identical to what mine had stock. The rest of the schematic is not relevant.

I think you saved a thumbnail of it.

[alchemy]

maybe this one will work...
Keep in mind this is not my amp, but an older Audio Guild amp that has an almost identical power supply and filtering as mine was stock...

[alchemy]

Still waiting on the guy to give me the voltage measurements, but I've made some progress. I couldn't find any zeners big enough locally, so I went with the 200ohm resistor for the time being. Now maybe I'm crazy, but to me I can hear a difference in the way it sounds when it's in the CT and when it's in the B+. CT sounds much better to me. So between that and a few other tweaks, I finally have this thing sounding pretty good. All except for the low E string. All notes higher have the most beautiful, shimmering, sparkiling clean sound, and when you start getting near the top of the volume range it starts to get some wonderful breakup. Only on the last little bit of volume travel does it really get mean, and it is an awesome rich distortion that reminds me a bit of a marshall 18 watt circuit I put in an old hifi amp. But the real low notes and chords leave something to be desired. It just seems like there is not enogh power to hold it together or something. It gets really muddy and farty sounding. My first thought was not enough filtering, but the first filtering stage is 80 mf, followed by 40 for the screens (and i think the vibrato is also on this node). Any suggestions on getting a more tight focused low end WITHOUT changing how the rest of it sounds? I've been playing with the preamp, but I don't want to alter anything too drastically as other than the low notes, this thing sounds amazing, like nothing I've ever heard!!!