Electrical HT characteristics of PTs used in the 45-50W amps, JTM to JCM eras

[Pete Farrington]

It's probably something of a niche interest I acknowledge, but I'd like to get a view on how the PT aspect of HT sag changed in the 45/50W models over the 3 decades of classic Marshall production.
So PT types RS, 118-55, 1202-55, 1202-118, 1202-133, 1202-164, 1202-324/x
As a bear minimum, the full end to end / red to red HT winding resistance would be really helpful.
Beyond that, the more data the better, eg
Primary resistance and nominal voltage setting.
Measured open circuit / unloaded HT VAC and the measured mains voltage.
Idle HT VAC and the output valve cathode current.
HT VAC at max unclipped output into rated load.
HT VAC at max overdriven output into rated load.

From my JTM45 based amp with Ingo Georges PT and an RS DL OT, I expect the RS and 118-55 240V primary resistance to be about 7 ohms, HT red to red HT resistance about 130ohms, 730VAC unloaded @ 240VAC mains
720VAC, 457VDC with the KT66 each drawing 30mA cathode current @ 241VAC mains
712VAC, 423VDC at max unclipped output, each KT66 drawing about 80mA cathode current @ 242VAC mains
702VAC, 397VDC at max overdriven output, each KT66 drawing about 120mA cathode current @ 242.5VAC mains

https://www.hammfg.com/files/parts/pdf/290MAEX.pdf

 

[tschrama]

Cannt help you with information about thise PT, but….From 720 Vac idle to 702Vac max overdriven…. Isnt that completely audible insignificant ?

 

[william vogel]

It's probably something of a niche interest I acknowledge, but I'd like to get a view on how the PT aspect of HT sag changed in the 45/50W models over the 3 decades of classic Marshall production.
So PT types RS, 118-55, 1202-55, 1202-118, 1202-133, 1202-164, 1202-324/x
As a bear minimum, the full end to end / red to red HT winding resistance would be really helpful.
Beyond that, the more data the better, eg
Primary resistance and nominal voltage setting.
Measured open circuit / unloaded HT VAC and the measured mains voltage.
Idle HT VAC and the output valve cathode current.
HT VAC at max unclipped output into rated load.
HT VAC at max overdriven output into rated load.

From my JTM45 based amp with Ingo Georges PT and an RS DL OT, I expect the RS and 118-55 240V primary resistance to be about 7 ohms, HT red to red HT resistance about 130ohms, 730VAC unloaded @ 240VAC mains
720VAC with the KT66 each drawing 30mA cathode current @ 241VAC mains
712VAC at max unclipped output, each KT66 drawing about 80mA cathode current @ 242VAC mains
702VAC at max overdriven output, each KT66 drawing about 120mA cathode current @ 242.5VAC mains

https://www.hammfg.com/files/parts/pdf/290MAEX.pdf

Pete, what’s your DC voltage at the center tap node when measured at idle, full rms clean and full power. It’s funny how the AC voltage holds fairly strong and the DC draws down more. I think the AC wave flattens on the peaks producing stronger rms numbers than what’s really happening. Chris Merren and I had a discussion about this but pertaining to mains input voltage. The rms numbers predict a greater voltage but because the peak voltage lowers it really effects the DC output.

 

[South Park]

A lot of transformer mojo has a lot to do with wire size and grade of wire .

 

[XTRXTR]

Interesting data request. I'll see what I can get on my 1202-324/7.
I am interested in getting more sag and was planning some experiments anyway. I need a baseline to see what if anything is working. Adding a resistor seems to easy and doesn't make linear sense. Hysteresis of a coil does. It may take a bit but I'll post what I have.

 

[Pete Farrington]

From 720 Vac idle to 702Vac max overdriven…. Isnt that completely audible insignificant ?

Fair point
I think that with the JTM45, the GZ34 kinda 'buffers' the HT winding from the increasingly heavy load presented by the power amp as output increases.
Also the 6k6 OT limits how much current the output valves draw from the HT supply.
My interest in this has been raised as I've built a 2204 / JTM mash up with EL34 into a 784-139 OT using a similar spec OT as I used with the JTM45, and I'm surprised by degree of the HT sag when using silicon rectification.

what’s your DC voltage at the center tap node when measured at idle, full rms clean and full power.

Post #1 updated

It’s funny how the AC voltage holds fairly strong and the DC draws down more. I think the AC wave flattens on the peaks producing stronger rms numbers than what’s really happening. Chris Merren and I had a discussion about this but pertaining to mains input voltage. The rms numbers predict a greater voltage but because the peak voltage lowers it really effects the DC output.

Great point, yes, the current draw on the winding will spike on the sine peaks, thereby tending to round off the waveform. I've thought before that a VAC peak meter would be handy for assessing amp power output, as Vrms is seems too affected by distortion. I'd try Vpeak reading off my digital scope, but its calibration / accuracy isn't great.

A lot of transformer mojo has a lot to do with wire size and grade of wire

Yes, certainly if either are inadequate, any mojo can go up in smoke

I'll see what I can get on my 1202-324/7

Thanks, no rush, but I'd certainly appreciate any info.

Adding a resistor seems to easy and doesn't make linear sense. Hysteresis of a coil does

My understanding is that PT transformer sag can be well approximated with resistors. Hysteresis of transformer grade silicon steel should be fairly low, and PTs have to be designed to operate well within mag circuit saturation, because if that occurs (eg mains voltage and loading both peak), primary inductance will collapse, fault current will be drawn and the mains fuse should blow.

 

[South Park]

Just like guitar pick ups you can use smaller wire and more of it to do the same job as bigger wire and less of it . With the choke and filter caps the transformer see no voltage spike in the power supple .

 

[XTRXTR]

Drake PT 1202-324/7 in a 2204 circuit
76 ohms red to red HT 2ndary
From blue common to:
2.8 ohm primary 120V winding black
8.4 ohm primary 220V winding yellow
7.4 ohm primary 240V winding orange

Wall Voltage: 122 Vac
Open circuit no tubes HT Vac measured at standby switch open circuit terminals: 728.8 Vac
Idle Tubes Russian 5881: plates 471Vdc, cathode 28mA, 13.2 watts 13.2/24=55%
Idle NO signal HT Vac with all tubes in place: 716 Vac
Max non-distorted 1kHz signal on output HT 703 Vac
Max clipped (square wave) 1kHz signal on output HT 676 Vac

I didn't take plate and cathode readings for Max non-distorted and Max clipped square wave. I was concentrating on the HT Vac I could do it tomorrow if you want them.

Lemme know and there you go.

 

[Pete Farrington]

Drake PT 1202-324/7 in a 2204 circuit ...

That's amazing, thanks
In the light of @william vogel 's insight, yes, the rectified VDC at the OT CT / HT fuse output at idle, max sine and max squarewave outputs, and the output valve cathode current would be useful.
Many TIA

 

[william vogel]

This is off topic, I realized the formula to wind my own version of the 1202-118/1202-55, I was able to expose the heater windings of an original. This established my turns ratio and wire gauge for the heaters. I then calculated the area available to fit the other windings in the space. It’s quite worrisome, if you run out of room, just throw it away. Anyway, I estimated, finding the largest wire possible for each winding (my primary has 3 different wire gauges as it moves through the taps). My DCR for the primary and secondary is nearly identical to XTRXTR’s measurements of the 1202-324.

 

[XTRXTR]

• Idle 471Vdc@HT fuse, 28mA cathode current
• Max sine sine 452 Vdc@HT fuse, 67mA cathode current
• Max square wave 412Vdc@HT fuse, 196mA cathode current

Cathode currents were within a few hundred micro amps of each power valve hence listing one.
Here are some other data points that may be useful

• OT CT: to red V4 pin3 43 ohms
• OT CT: to white V5 pin3 42.5 ohms
• OT CT: V4 pin3 Vdrop Power@Idle 1.032Vdc
  
• OT CT: V5 pin3 Vdrop Power@Idle 1.031Vdc
  
• OT CT: V4 pin3 Vdrop Power@Max Sine 2.601Vdc
  
• OT CT: V5 pin3 Vdrop Power@Max Sine 2.521Vdc
• OT CT: V4 pin3 Vdrop Power@Max Square 5.742Vdc
• OT CT: V5 pin3 Vdrop Power@Max Square 5.692Vdc

 

[Pete Farrington]

dle NO signal HT Vac with all tubes in place: 716 Vac
Max non-distorted 1kHz signal on output HT 703 Vac
Max clipped (square wave) 1kHz signal on output HT 676 Vac

• Idle 471Vdc@HT fuse, 28mA cathode current

• Max sine sine 452 Vdc@HT fuse, 67mA cathode current

• Max square wave 412Vdc@HT fuse, 196mA cathode current

Thanks a million, that’s great info!
I suggest not to probe a cranked output valve anode though; I suppose a resistive load may have prevented voltage spikes being too excessive. With an inductive load, the meter may have been damaged.

So idle rectification ratio is 0.66
Max sine is 0.64
Max square is 0.61
Some of that will be due to ripple, but given 100uF reservoir, I guess that a fair chunk of it will be due to the rms conversion in the meter giving an unhelpful impression that the VAC is holding up better than it really is.

 

[XTRXTR]

I definitely used my dummy load. Had to work fast due to heat. I did not notice any spikes on the oscope when probing with the meter.

I made the mistake of using a cabinet once a long time ago, the scope probe must have had a charge, and pow! I nearly fell over. I had to check I was still alive, my underwear etc. Now I have a rule and a check list I go through

Not sure I understand about the rms of the meter, what line of thought are you writing?

 

[Pete Farrington]

To put another perspective on it, if the waveform of HT winding is scoped, as the current draw of the power amp increases (eg from idle to max sine to max square), the HT wave shape will probably change from being a good sine wave to being rounded off. Like the signal output of an AC30. As if the amp is clipping the transformer HT winding output.
The rms value of the rounded off waveform is little different to the full sine, but the reduction of the vpeak level reduces the rectified HT VDC, because with a cap input rectifier, the VDC = Vpeak of the VAC input to the rectifier.
Hope that made sense

 

[XTRXTR]

In fact I had the HT scoped originally and you could see that rounding of the peak you are speaking of comparing the 122VAC mains to the HT VAC secondary. You can see it lose amplitude and the shape just like you describe looks similar to the beginning of tube clipping where the peak is just beginning to distort but still has a sine shape.

Plumbing analogy
Like a soft hose with plenty of pressure to keep the flow, but take that same hose and use it as a siphon and it collapses as the draw becomes larger than the pressure on the inlet.

With the HT winding the wave form collapses (a bit) as the current draw is closer to exhausting the HT secondary ability to provide it.

 

[arthur.lowery]

It's probably something of a niche interest I acknowledge, but I'd like to get a view on how the PT aspect of HT sag changed in the 45/50W models over the 3 decades of classic Marshall production.
So PT types RS, 118-55, 1202-55, 1202-118, 1202-133, 1202-164, 1202-324/x
As a bear minimum, the full end to end / red to red HT winding resistance would be really helpful.
Beyond that, the more data the better, eg
Primary resistance and nominal voltage setting.
Measured open circuit / unloaded HT VAC and the measured mains voltage.
Idle HT VAC and the output valve cathode current.
HT VAC at max unclipped output into rated load.
HT VAC at max overdriven output into rated load.

From my JTM45 based amp with Ingo Georges PT and an RS DL OT, I expect the RS and 118-55 240V primary resistance to be about 7 ohms, HT red to red HT resistance about 130ohms, 730VAC unloaded @ 240VAC mains
720VAC, 457VDC with the KT66 each drawing 30mA cathode current @ 241VAC mains
712VAC, 423VDC at max unclipped output, each KT66 drawing about 80mA cathode current @ 242VAC mains
702VAC, 397VDC at max overdriven output, each KT66 drawing about 120mA cathode current @ 242.5VAC mains

https://www.hammfg.com/files/parts/pdf/290MAEX.pdf

Would reply but I have had to much wine with dinner to mess with HT! I will put it n my to do list!

 

[arthur.lowery]

So, The measurements on the secondary of the 1202-128 mains transformer (no 5V tap) are 39.1 and 41.3 ohms (each secondary to centre tap). The primary is 9.3 ohms, including the fuse, at a 240 V setting. This is a 50W Black Flag. Actually, I see you didn't want that MPT! I'll do some other measurements if you still want them.

 

[Pete Farrington]

The measurements on the secondary of the 1202-128 mains transformer (no 5V tap) are 39.1 and 41.3 ohms (each secondary to centre tap). The primary is 9.3 ohms, including the fuse, at a 240 V setting. This is a 50W Black Flag

Great, thanks, the lower HT winding resistance may indicate that there was a complete redesign for that mains transformer, they didn’t just ‘not bother’ with the 5V winding. I’m surprised such a redesign occurred so early.

This is a 50W Black Flag. Actually, I see you didn't want that MPT! I'll do some other measurements if you still want them.

Oops, I just listed every ‘50 watter’ mains transformer code I was aware of, unfortunately my knowledge isn’t comprehensive. Black flag era is definitely of interest, all such info gratefully received

 

[arthur.lowery]

The transformer I have was probably installed in 1970 (or 71). The inside of the chassis has condensed varnish from the previous transformer at that end, which left a shadow where the internal filter cap used to be.
I see your pint that leaving out the 5V winding may have given a bit more space for a heavier-gauge HT winding, hence the lower resistance! I resume the turns-per-volt may have been the same (I seem to remember that turns per volt was a starting point for a design - I should check that).

 

[arthur.lowery]

Some results (8 ohm load) 1 kHz sine in, Presence low
HT off Mains = 235V; Sec = 321 Vrms; B+ = 436 Vdc
HT on Mains = 235V; Sec = 317.2 Vrms; B+ = 415 Vdc I = 79.4 mA (no signal)
HT on Mains = 235V; Sec = 305 Vrms; B+ = 377 Vdc I = 280 mA (50 Vpp sine out scope , meter says 18.2 Vrms, 41.4 W)
HT on Mains = 235V; Sec = 301 Vrms; B+ = 364 Vdc I = 330 mA (50 V p-p trapezoid- as square as I could get, some asym crossover distortion, meter says 21.9V rms, 60W assuming rms is OK)

Better Square (bass, mid, presence full, treb min*):
HT on Mains = 233V; Sec = 299 Vrms; B+ = 356 Vdc I = 370 mA (49 V p-p square on scope = 70 W, meter says 23.3V rms => 68W assuming rms is OK for squares (seems to be!))

(JTM Black Flag SN10348 with ss rectifier added about 1970 and new MPT at that time 1202-128)

Caveat - I have a 10 ohm resistor to the HT fuse (post the first cap. I made these measurements after this, so they are 10*I below what they should be), to measure the bias.

*There is an amazing variety of waveforms available when the tone-stack interacts with the PI. Have people posted these on the forum?