Just drew out a schematic for my JTM45 build

[mort]

I've followed the circuit, double checked the values that were used, and created a schematic that exactly matches how this JTM45 was built.


This is basically an old school JTM45 circuit with an adjustable bias added.



Should this schematic work as it is drawn?

[eschertron]

I've followed the circuit, double checked the values that were used, and created a schematic that exactly matches how this JTM45 was built.


This is basically an old school JTM45 circuit with an adjustable bias added.



Should this schematic work as it is drawn?

Good looking schem. I'd like to see PT voltages and OT impedances to complete the picture for me. There's something fishy about the input section, though. R8 and R11 will be shorted if you don't plug into the right jack. Is that the design?

[pdf64]

Of the resistors labelled 1 watt, probably the only one that needs to be is R30.
The V1 cathode resistor is usually 820 ohms, rather than 920.
That implementation of the standby switch is likely to cause excessive stress to the rectifier / premature failure, due to 'hot' switching of a capacitive load; if it's really necessary, better to put it 'after' the reservoir cap or (my preference) on the screen grids.
There's usually a shared 1k resistor in the screen grid supply for JTM45 with KT66.
Pete

[mort]

Ok here's an amended drawing with PT, OT and choke info.

The input design is taken straight from the original schematic. Each channel has a hi and lo input http://www.pleximods.com/images/66JTM45Schematic.gif

[mort]

Of the resistors labelled 1 watt, probably the only one that needs to be is R30.
The V1 cathode resistor is usually 820 ohms, rather than 920.
That implementation of the standby switch is likely to cause excessive stress to the rectifier / premature failure, due to 'hot' switching of a capacitive load; if it's really necessary, better to put it 'after' the reservoir cap or (my preference) on the screen grids.
There's usually a shared 1k resistor in the screen grid supply for JTM45 with KT66.
Pete

Down at the surplus store 1W resistors are very inexpensive so I just used them.

Will there be any noticeable differences between an 820 and 920 resistor? I'm not sure how the 920 got in there...

So the stand by should go between the choke and the pair of 470 ohm resistors? (just realized I didn't number those resistors)


I noticed while I was mapping it out that the shared 1K wasn't in my build... effects it would have with/without??

edit: I see why I left that 1K shared resistor out. I was using Weber's layout as an assembly guide and they don't use it. I wonder why they omitted it ??

[pdf64]

'Down at the surplus store 1W resistors are very inexpensive so I just used them.'
That's fair enough, I was thinking it was a specification rather than a record of what was used for your build.

I doubt that there would be any noticeable differences between an 820 and 920 V1 cathode resistor.

'So the stand by should go between the choke and the pair of 470 ohm resistors? (just realized I didn't number those resistors)'
Yes. It doesn't result in a total mute unless a 2 way switch is used to ground the screens - a minuscule signal gets through.

The shared 1k screen grid resistor will reduce the screen grid dissipation and affects the large signal output waveform. It's subtle, I haven't A/B'd it to check if its noticeable.
Pete

[mort]

OK thanks. Will definitely relocate the stand by.



Is it normal, with only the rectifier tube installed, for the B+ directly off of the rectifier to jump from 369vdc (in stand by mode) to 527vdc (in play mode)?

[NZaudio]

I have schematics for it but they are too big to insert here.
send me your e-mail then I can post them if you like

[pdf64]

'Is it normal, with only the rectifier tube installed, for the B+ directly off of the rectifier to jump from 369vdc (in stand by mode) to 527vdc (in play mode)?'
Yes, as drawn, the standby VB+ doesn't have a reservoir cap, so the Vdc will be an average of the full wave rectified Vac. Play mode (with reservoir cap etc connected but current draw) it will be the peak value.
The unloaded transformer secondary voltages will be higher than the loaded voltages, by 5 - 15%.
Pete

[mort]

K, thanks. I have so much to learn but I'm getting there

[mort]

Another general question for you guys... If I remove the power tubes, I'm getting 45.4vac on the bias tap. When I measure with the power tubes in, I'm getting 35.4vac out of it. That seems like a pretty big drop to me. What say ye?

note: measurements taken with amp on a 75w bulb limiter

[pdf64]

My guess is that the Vac across the power transformer primary changes by a similar proportion under the two conditions.
This is due to the light bulb limiter dropping voltage when the power tubes are drawing current.
Maybe consider a higher power light bulb, now that the possibility of severe shorts / errors has been greatly reduced; that avoid it dropping so much voltage under normal running conditions.
Pete

[Pedro Vecino]

Driver output capacitors are marked as 0.01uF (10n) in your schematic. Originals are 0.1uF (100n).
Older models (with 430/440V in the plates) usually have a 1K resistor between the choke and the screen grid resistors (in series).

[mort]

nice catch on the cap value error. It's been edited


The missing resistor had been pointed out. This circuit having been built from both the Weber circuit and the original circuit doesn't have it. I plan to try with and without and see what changes. That is, after I figure out why it's blowing fuses :/

[Pedro Vecino]

Later editions Marshall are with a lower voltage and 5881 (without that resistor) but many units of the old 60īs with KT66 uses it.
The oldest scheme is this: http://www.raw-sewage.net/images/jtm45orig-schm1.jpg

Some details about the JTM45:
- The ideal voltage with its low filtering (32/32/16/16) and KT66 is in my opinion 430/440V after the GZ34.
- The impedance of the output transformer primary with KT66, between 7 and 8K.
- 270K resistors at the output of the potentiometers offer poor isolation between channels. Best use 470K and appropriate capacitor value in brilliant channel.
- Separating the cathodes in the first 12AX7 provides flexibility to find the best balance between channels.

This is the last I built:
http://www.bymcomunicacion.com/pvecino/JTM/5.jpg