Manzamp!

[talbany]

So what is Mr D to do?

A common alternative to split first position caps is to separate them with some low value resistors 10-33 R.,

Good Luck

Tony

[ic-racer]

Update:

Dropping string 2k2, 22k, 5k
Rectifier: 5U4GB with safety diodes (no more sparks!! )
EL34 cathode current 20ma
EL34 Plate 470
EL34 Screen 468

PI: 326 & 337

V2: plate 190
Cathode 184

V1: 242 & 250

It will be a major overhaul to change out the green 20w ceramic resistors so I have left them for now. I misread the label but they are 5r not 6r. I'll update the schematic eventually, but now it is time to plug in and play...

[martin manning]

Can you get the DC resistance of the PT primary and secondary? And, is the PT Classic Tone 40-18073? The voltage above seems high for a 325-0-325 secondary.

[ic-racer]

Can you get the DC resistance of the PT primary and secondary? And, is the PT Classic Tone 40-18073? The voltage above seems high for a 325-0-325 secondary.

Yes 40 18073.
The secondaries were 49 r and 32 r

[talbany]

JJ 34's love 470v

You have to watch those rather high PI voltages though..Anything over 310 can tend to generate some rather nasty overtones when you start to drive the output section into distortion..Looks like if you drop the voltage at the PI down to 300v everything else should line up perfectly.. On the other hand if you like the way it sounds now...Let it Eat!!
Weird that it went from 520V to 470v going to the 5U4..50V

BTW I prefer those JJ 34's w/470 on the plates right around 65% or 34ma cathode current..

I have rambled enough on this amp!!

Great job BTW!!... Outta here!

Tony

[martin manning]

Yes 40 18073.
The secondaries were 49 r and 32 r

How about the primary?

[trane34]

Really cool scratch build. Sounds really good too. I couldn't help but think that a Marstran Radiospares OT would work really well in it.

[groovtubin]

JJ 34's love 470v

You have to watch those rather high PI voltages though..Anything over 310 can tend to generate some rather nasty overtones when you start to drive the output section into distortion..Looks like if you drop the voltage at the PI down to 300v everything else should line up perfectly.. On the other hand if you like the way it sounds now...Let it Eat!!
Weird that it went from 520V to 470v going to the 5U4..50V

BTW I prefer those JJ 34's w/470 on the plates right around 65% or 34ma cathode current..

I have rambled enough on this amp!!

Great job BTW!!... Outta here!

Tony

The 5U4 can source more current than a GZ34, but it's internal resistance is higher than a GZ34 dropping more voltage. At idle you'll see something like 20 to 40 volts different depending on you bias points/idle current. At full pwr you'll see a little more voltage drop with the 5U4, but it really depends on your bias points as to how much. The closer you get to constant current the less drop you'll see with either tube.
The main thing to watch for is the current ability of the 5vac winding. The 5U4 draws a little over 1 amp more than the GZ34, and alot of the trannies made for a GZ34 will not have the winding over rated enough to run the 5U4 without burning up. PaulC

jim

[ic-racer]

Thanks for all the comments.

The PT does put out 3A, just enough for the 5U4.

Tony, I did hear some oddball overtones when overdriven, will think more about the resistor dropping chain tomorrow and the PI voltages.

[ic-racer]

Weird that it went from 520V to 470v going to the 5U4..50V

Also changed from 22k, 10k, 5k to 2k2, 22k, 5k when I changed the tube.

[talbany]

Weird that it went from 520V to 470v going to the 5U4..50V

Also changed from 22k, 10k, 5k to 2k2, 22k, 5k when I changed the tube.

Ahh!! That makes some sense..Thanks for the sanity check!

Would love to play that amp sometime!!

Again Great work!!

T

[ToneMerc]

Can you get the DC resistance of the PT primary and secondary? And, is the PT Classic Tone 40-18073? The voltage above seems high for a 325-0-325 secondary.

MCI power transformers can be all over the place for both the Fender and Marshall winds, too hit and miss for my tastes.

No 120V primary 325-0-325 PT should result in 520 volts on the plates loaded with a GZ34.


IC, if you replace the PT and its back down the 435V range with the new iron, won't have redo the string mods?

TM

[martin manning]

I think It'd be a good idea to pull the rectifier tube out and measure the PT DC resistances and unloaded voltages. There's quite a spread in the secondary resistances you posted, so taking another reading to confirm them would be good.

If I guess at the PT primary resistance (and the primary is significant in the Rs calculation) it seems like there is enough series resistance for a 5AR4/GZ34, and certainly enough for a 5U4GB, but it'd be good to know exactly where you are. I wonder about the purpose of the series resistors then-are they there to protect the rectifier or lower the B+? Or both?

I guess you don't really know which rectifier tube Dumble used in the originals, and how do you know that the reservoir is 110uF? Just as a side note, the TW Rocket gets away with using a GZ34 with an 80uF reservoir and no added series resistance.

[ic-racer]

I think It'd be a good idea to pull the rectifier tube out and measure the PT DC resistances and unloaded voltages. There's quite a spread in the secondary resistances you posted, so taking another reading to confirm them would be good.

Re-measured multiple times and they are curiously different. I had blown 3 fuses in a row and suspected I fried the transformer, or shorted a few windings together when the Gz34 gave a little fireworks show. Since then, however, it was played for over 4 hours yesterday with the 5u4gb and protector diodes and no issues.

I wonder about the purpose of the series resistors then-are they there to protect the rectifier or lower the B+? Or both?

Yes, still considering the idea of using those resistors to drop B+

I guess you don't really know which rectifier tube Dumble used in the originals

Check out the picture what do you think? Looks like 5u4gb to me, but I did find some pictures of RCA GZ34 in a bigger bottle but have not seen one in person.

, and how do you know that the reservoir is 110uF?

Again check out the picture. My interpretation is that those first two big radial capacitors would be similar in value to the first two large axial capacitors of the 'usual' Precision Power supply. So 220uf + 220uf = 110uf. So I think he designed the rectifier circuit with that value in mind. So he increased the series resistance and or added helper diodes on the socket (which would not be seen in the picture) so the rectifier tube could handle it. What do you think?

[martin manning]

Yes, those first two radial caps do look big enough to be 220uF. Your schematic shows a GZ34, so that's what I assumed you assumed. I agree the bottle in the picture does look like a 5U4. The 5U4 has both a lower series resistance requirement and a larger voltage drop, so the purpose of the series resistors is still a bit mysterious.

Did you get a primary resistance for the PT?