Mojotone 18w TMB experiences?

[Explorerman]

regarding the diode rectifier,have a look at this schematic,he's used 5W resistors prior to the diodes to drop the 290-0-290 HT down,i'd be interested in opinions on this.

From what i’ve been reading, the higher the resistance value in the B+ line, the more sag you get. I want the option to switch between the tube rec for some sag and the ss rec for a tighter response that’s why I was trying to keep the resistance value low.

[geoff 1965]

yeah i get you! i'm working on a similar idea myself with a 6V6 amp,i just wanted any opinion on the safety aspect of the resistors i have'nt seen them before.

[colossal]

Although those 47R 5W are being used to shoehorn the wrong PT for the job into that build, if you look at a datasheet for a GZ34, there is recommendation of added series resistance with the secondary to limit peak charging current. Almost no guitar amp manufacturer of yore did this; likely yet another cost cutting measure. In low plate voltage, high (almost constant current) amps like the AC30, the peak charging current often exceeds the rectifier's limit, so added series resistance with the secondary is a good idea. And especially when a rectifier is facing a high amount of capacitance (64uF).

Also, resistors do not behave non-linearly like a tube rectifier. Some may argue that the response is similar enough or indistinguishable. YMMV and all that. I built a very high gain EL84 based amp once. Ran it with a GZ34 and it felt and sounded magnificent. Chewy and thick with monstrous bloom and sustain. Switched to solid state rectification and bye bye feel and sound.

[geoff 1965]

good info! i'm doing the same as explorerman except i'm going from the PT to a dpst switch then to the ez81/diodes small sag.not bothering with resistors or zeners to drop the B+,just going to hit the tubes with the higher voltage with some adjustment to filtering&droppers.

[crgfrench]

EZ81 will offer more sag than a GZ34.

[geoff 1965]

here's where i think explorerman will have problems with the diode rectifiers;
stock TMB ez81 will give him about 345 on the el84 plates but the diodes will give him about 375,the higher voltage is'nt the problem it's the droppers between the power & preamp i.e. the TMB has 8K2 and 2K2 droppers between the power and preamp whereas the 20W has 10K and 10K to bring the preamp voltages down.
if he swaps the 2K2 for a 10K to bring the diode B+ down it's going to drop the voltage too much for the ez81 circuit which in turn is going to alter the character! one option would be to adjust the VVR so it drops the preamp as well and keep the stock resistors.
more food for thought!

[JMPGuitars]

And especially when a rectifier is facing a high amount of capacitance (64uF).

The rectifier is only facing a maximum of 32uF. The cap cans aren't wired in parallel.

[colossal]

And especially when a rectifier is facing a high amount of capacitance (64uF).

The rectifier is only facing a maximum of 32uF. The cap cans aren't wired in parallel.

Sorry, I should have been more clear. I was referring to the 6V6 Plexi type schematic posted earlier. It is using a paralleled 32-32uF can for 64uF after a GZ34. Even though the capacitance is high, it is reasonable as two 6V6s is a small load for a GZ34. The series peak limiting resistors are a good part of any design.

[JMPGuitars]

Sorry, I should have been more clear. I was referring to the 6V6 Plexi type schematic posted earlier. It is using a paralleled 32-32uF can for 64uF after a GZ34. Even though the capacitance is high, it is reasonable as two 6V6s is a small load for a GZ34. The series peak limiting resistors are a good part of any design.

Ah, interesting. That is a funky schematic.

[geoff 1965]

too funky for my skill level Josh at the moment,but one day!
here's another idea relating to explorerman's post,look how trinity have the VVR dropping the whole amp voltage but only on the diode rectifiers.

Trinity18_Plexi_SL_Schematic.jpg

[Explorerman]

Ok, so, I’ve made some progress into the build, only thing left to do is drop in the populated turret board!

I’ve powered up the AC side only so far up to the EZ81 with the rest of the power rail disconnected. On pin 3 of the EZ81 i’m getting 278VDC. Is this high? Do I continue on and connect up the rest of the circuit and see what I get at the EL84s?

With the SS rec (just the 2 x 1N4007 at this stage) I get 273VDC (lower than I expected).

[colossal]

Something is amiss. Your rectified voltage is way too low and especially if there were no load on the amp.

[Explorerman]

You are right, I’ve got 600vac to pins 1 and 7 (both sides are 300v to ground). The heater pins 4 and 5 I have 6.5vac. Could it be a faulty rec tube?

[colossal]

You are right, I’ve got 600vac to pins 1 and 7 (both sides are 300v to ground). The heater pins 4 and 5 I have 6.5vac. Could it be a faulty rec tube?

Your AC Vrms voltages are within spec as is your filament voltage. Unloaded, you would see:

VACrms = VDCpeak/sqrt(2)
VDCpeak = 300VAC * 1.414
VDCpeak = 424VDC

Do you have another tube to try?

[Explorerman]

No, I dont have another tube, though its really strange that I'm getting the almost the same voltage with the SS rec (regardless whether the EZ81 is removed or not). One side of the rec is 600VAC, the other side is 275vdc. I'm going to try another Fluke MM today. (yes I've checked the polarity of the two diodes).

[Explorerman]

Update: Another Fluke Meter gives me the same results, I’m at a loss as to how this can be.

[Explorerman]

I took a scope reading:

Channel’s A and C are straight off the PT HV (EZ81 pins 1 and 7), channel B is the Recified output DC (pin 3). Do I have a faulty PT?

[colossal]

Hmmm....

Can you post a photo of your rectifier tube and the leads connected to it? That looks like the transformer is operating in the "additive" mode and it's possible they got the color code wrong with the windings. Heyboer winds transformers for Mojo (there should be a number like HTS-xxxxx on the sticker identifying the PT). If you can, I would desolder the leads from the tube socket and test them individually (energized/floating) with your scope and document again. Do this for each lead so that you can identify the voltage and the polarity of the waveform. You might think about sending that scope capture above ^^ to Phil at Heyboer.

[Explorerman]

The below one is with no rec connected, just the PT red leads on the tube socket with nothing connected, essentially floating with the centre tapped still connected to ground, no difference.

[colossal]

You should only have four connections to the rectifier socket from the power transformer:

0VAC Filament (pin 4)
6.3VAC Filament (pin 5)
290VAC Secondary (pin 1)
290VAC Secondary (pin 7)

You will probe Pin 3 for your rectified B+