Pleasantly Surprised By Transformers

[NewReligion]

Perhaps the following has been discussed but I had a pleasant surprise.

(Discussed at End-Transformer Content).

I bought a ”Rescue” Marshall as I refer to the vintage amps that have been drilled on etc…

This is my 3rd from 1976. I have a line on a set of OEM black bat toggles but I’m not in any hurry there as I installed a matching set of metal toggles.

The amp was/is a Marshall Model 1987. The iron is all stock (This is the main subject). The faceplate was wasted in addition to someone drilled a 3/4” hole to install a 12VAC Fender style Indicator Light. All the pots remarkably are intact & functional.

I rewired the 120VAC/Switches/Mains Fuse/PT network. Replaced the faceplate & installed a 120VAC Marshall Indicator Light.

I completed my new “Divergent” circuit. The circuit signal sounded & felt weak. That is to say not making the cut.

I always fine tune every amp I work on but this was where it became interesting to me. I went ahead & installed a buffered loop. Then, in order to start tuning the amp I checked the bias which I normally do first thing.

When checking the VDC on the Mains Fuse & JJ E34L pin 3 I got 400VDC under load at idle, all volumes at zero. I thought this didn’t seem correct for a 1976 50 watt. I checked the numbers on the PT & OT.

I was pleasantly surprised to discover the Drake set to be a PT 1202-164 & the OT a 784-139. These Transformers were used in “US” amps from 1969-1973. The UK PT in 1969 was a lay down.

I can understand how Jose Arredondo liked the higher voltage of 500VDC for his clipping circuits but this is a different animal. No clipping diodes or transistors, no cathode follower, four stages of gain, nice & brown.

If anyone has additional information to add, challenge or correct, I am open to it as I am about to start assembling a list of Marshall Transformer data.

I did go back in & add dampener/swamp resistors. The Screen resistors on the power tubes are 5 watt 1k.

I did not add any type of Resonance mod. The NFB is 100k on 8 ohm tap.

Feel free to take this any where you like.

I would really like to hear or should I say read more data on the Drake & Dagnall Iron sent to the US from 1969 to 1981.

Thanks, David

 

[mickeydg5]

So what is the question?
You seem to have an amplifier with a PT primary at least at 10%+ low.
Is there anythiong else?
Hotter is usually better but you can make due with what you have.

An amplifier is an addition of all of its sums. Those sums may have to be boosted for the final number.

 

[NewReligion]

So what is the question?
You seem to have an amplifier with a PT primary at least at 10%+ low.
Is there anythiong else?
Hotter is usually better but you can make due with what you have.

An amplifier is an addition of all of its sums. Those sums may have to be boosted for the final number.

EDIT;

Not so much a question but was looking for others who may have come across a Marshall from 1976 or later with what seems to me to be a low VDC.

I’m finding this is not unusual as I have a 2204 from 1976 that measures 364VDC with 6.124VAC on the Heaters.


I have yet to confirm the higher VDC on this model of PT.

I thought 300VDC - 500VDC was quite a large allowance in tolerance.

I was looking for other techs thoughts on the lower VDC.

Not sure why this fact went over my head with all of the vintage Marshall’s over the years.

My data compilation of personal Marshall’s is in progress.

Thank you for joining in. I wish to organize a Dagnall/Drake data collection library down to actual voltages & wire color coloring to include of course models & years,

Perhaps the will be better when started fresh on a new thread with that intended purpose.

David

 

[Pete Farrington]

When checking the VDC on the Mains Fuse & JJ E34L pin 3 I got 400VDC under load at idle, all volumes at zero. I thought this didn’t seem correct for a 1976 50 watt.

HT fuse!
This topic crops up fairly regularly.

My guess is that Marshall purposely reduced the HT voltage and hence power output of the 50W chassis of the 70s to allow them to be used in 2x12” combos using a pair of 25W greenbacks. ie without blowing them up when cranked.

See this complication of Marshall voltage charts. It’s provenance is unknown, my guess is someone may have copied and pasted exerts from official service info from the mid 70s. The point being that your voltages are normal for 50 watters (35W really) of the 70s.

 

[NewReligion]

HT fuse!
This topic crops up fairly regularly.

My guess is that Marshall purposely reduced the HT voltage and hence power output of the 50W chassis of the 70s to allow them to be used in 2x12” combos using a pair of 25W greenbacks. ie without blowing them up when cranked.

See this complication of Marshall voltage charts. It’s provenance is unknown, my guess is someone may have copied and pasted exerts from official service info from the mid 70s. The point being that your voltages are normal for 50 watters (35W really) of the 70s.

Excellent data my friend, thank you.

I’m finding a second of my 50 watt (2204) from 1976 PT is also a 1202-164. It measures 364VDC with AC Heaters at 6.124VAC.

I really appreciate the data table. Looks like I will be noting some data on my Marshall’s. A simple compilation at first.

Thanks again @Pete Farrington

David

 

[neikeel]

We’ve talked about this lots in the vintage and workshop before, your findings are entirely normal.
69 stand up -420 (same as a -118)
Early 70s 400v
Late 70s 380 thereabouts

 

[NewReligion]

We’ve talked about this lots in the vintage and workshop before, your findings are entirely normal.
69 stand up -420 (same as a -118)
Early 70s 400v
Late 70s 380 thereabouts

@neikeel

Thank you. That is consistent with the 50 watt Drakes I have. I still need to go over the 100 watt Dagnall’s.

I eventually wish to complete a comprehensive collection of data on all Marshall used Drake/Dagnall’s even to include the reissue’s.

Such a publication may be useful for some.

I will start a new thread in time when I am prepared to move forward. I plan to include all Heater VAC, PT VDC as well as wire color coding. Perhaps additional data.

If one exists it sure would save some time. Data Sheets a great if found but hands on is just as important.

Thanks again.

David

 

[neikeel]

Good idea
Ask Ned B he has good info
There was a thread from George M (sticky?) on the subject. I’ll have to look later.

 

[NewReligion]

Good idea
Ask Ned B he has good info
There was a thread from George M (sticky?) on the subject. I’ll have to look later.

Excellent & thank you. Noted.

David

 

[Pete Farrington]

Perhaps additional data

I think including the end to end resistance, and open circuit (ie totally unloaded) V AC of the PT’s HT winding would be really useful, as it gives a strong indication of how saggy the HT supply will be at high output levels

 

[TAZIN]

1202-55; intro date 09/64; approximately 450Vdc plate voltage.
1202-133; intro date late 1966; apprx. 475Vdc rectified voltage.
1202-118; intro date 02/67; apprx. 420Vdc plate voltage.
1202-164; intro date 04/69; apprx. 420Vdc plate voltage.
1202-164; revision 1; 1972 - 1980 ??; apprx. 390-400Vdc plate voltage.

EDIT: 1202-164 Rev.1 raised to 390-400Vdc due to low voltage reports based on 120Vac input tap which only produced heater voltages in the 5.8 - 5.9Vac range on early units. Correcting the heater voltage to 6.3 - 6.4Vac (through use of variac or 110Vac input tap) produces plate voltages within the 390-400Vdc normally when the idle plate dissipation is 65-70%.

 

[NewReligion]

I think including the end to end resistance, and open circuit (ie totally unloaded) V AC of the PT’s HT winding would be really useful, as it gives a strong indication of how saggy the HT supply will be at high output levels

Great thought. Thank you.

David

 

[NewReligion]

1202-55; intro date 09/64; approximately 450Vdc plate voltage.
1202-133; intro date early 1967; apprx. 475Vdc rectified voltage.
1202-118; intro date 02/67; apprx. 420Vdc plate voltage.
1202-164; intro date 04/69; apprx. 420Vdc plate voltage.
1202-164; revision 1; 1972 - 1980 ??; apprx. 390-400Vdc plate voltage.

EDIT: 1202-164 Rev.1 raised to 390-400Vdc due to low voltage reports based on 120Vac input tap which only produced heater voltages in the 5.8 - 5.9Vac range on early units. Correcting the heater voltage to 6.3 - 6.4Vac (through use of variac or 110Vac input tap) produces plate voltages within the 390-400Vdc normally when the idle plate dissipation is 65-70%.

@TAZIN

Thank you for the collected data. This is a great start to the data collection.

David

 

[TAZIN]

Here's some more data for you...
RS potted military PT; intro date around 09/1965; apprx. 520-540Vdc plate voltage.
1204-43; intro date 04/1966; apprx. 560Vdc plate voltage.
1203-80; intro date 02/1967; apprx. 480-500Vdc plate voltage.
1209-3; usage '70-'72; apprx. 500-510Vdc plate voltage.
T2562; intro date around 08/1967; apprx. 470-500Vdc plate voltage (490Vdc avg.).
T2623; intro date 04/1969; apprx. 480-500Vdc plate voltage (490Vdc avg.).

 

[NewReligion]

Here's some more data for you...
RS potted military PT; intro date around 09/1965; apprx. 520-540Vdc plate voltage.
1204-43; intro date 04/1966; apprx. 560Vdc plate voltage.
1203-80; intro date 02/1967; apprx. 480-500Vdc plate voltage.
1209-3; usage '70-'72; apprx. 500-510Vdc plate voltage.
T2562; intro date around 08/1967; apprx. 470-500Vdc plate voltage (490Vdc avg.).
T2623; intro date 04/1969; apprx. 480-500Vdc plate voltage (490Vdc avg.).

Thank you so much. If many get involved over time it can all come to get her faster.

I likely will publish what I/We have then continue to add on as time progresses after the initial launch so to speak of the project.

I just think it would be helpful to have a source anyone can find online.

Thanks again.

David

 

[mickeydg5]

Excellent data my friend, thank you.

I’m finding a second of my 50 watt (2204) from 1976 PT is also a 1202-164. It measures 364VDC with AC Heaters at 6.124VAC.

I really appreciate the data table. Looks like I will be noting some data on my Marshall’s. A simple compilation at first.

Thanks again @Pete Farrington

David

What data table?
That is a common pdf found on the internet.
Did you not have that vintage Marshall Voltage Chart document?
Copy it and keep it in your records.

 

[mickeydg5]

If looking at Classictone transformer information one can see which Dagnall and Drake models were used with which models in which eras.
Classictone did a fairly decent job of documenting and copying/replicating those things.

 

[NewReligion]

If looking at Classictone transformer information one can see which Dagnall and Drake models were used with which models in which eras.
Classictone did a fairly decent job of documenting and copying/replicating those things.

You are correct I did not have that data. I am also accomplishing hands on testing of PT’s & OT’s.

I was aware Classic Tone claimed to have followed exact color coding. The 10-15 I worked with did indeed meet that criteria. Thank you for the suggestion.

I do/did like the Classic Tone iron. I am measuring the OT Primaries for example. I think it is important to do as much hands on as possible even within the same models as I have a 36VDC difference per side on two OT exact models from the same year.

I have new 100 watt Heyboer clones of Dagnall’s. The Heyboer ohm reading on the Primaries is 30% higher than the actual 1974/76/80 100 watt Dagnall’s I have.

Thanks again for stepping up.

David

 

[mickeydg5]

You are correct I did not have that data. I am also accomplishing hands on testing of PT’s & OT’s.

I was aware Classic Tone claimed to have followed exact color coding. The 10-15 I worked with did indeed meet that criteria. Thank you for the suggestion.

I do/did like the Classic Tone iron. I am measuring the OT Primaries for example. I think it is important to do as much hands on as possible even within the same models as I have a 36VDC difference per side on two OT exact models from the same year.

I have new 100 watt Heyboer clones of Dagnall’s. The Heyboer ohm reading on the Primaries is 30% higher than the actual 1974/76/80 100 watt Dagnall’s I have.

Thanks again for stepping up.

David

I am not saying all replications or manufactures are exact, but can you explain the 36VDC per side difference scenario?

A company can duplicate a product but do it in a different manner from the original....in that case the Heyboer will not match the DCR of the original. 30% higher resistance tells me they used more windings but kept within tolerances using the same approximate target bandwidth. Different wire diameter and characteristics?

 

[NewReligion]

I am not saying all replications or manufactures are exact, but can you explain the 36VDC per side difference scenario?

A company can duplicate a product but do it in a different manner from the original....in that case the Heyboer will not match the DCR of the original. 30% higher resistance tells me they used more windings but kept within tolerances using the same approximate target bandwidth. Different wire diameter and characteristics?

Certainly. I measure the resistance between the anodes (pin 3) without tubes & Power off.

It may be more accurate to apply voltage to the plate to plate and measure voltage off of the secondary impedance tap.

I am simply interested in better understanding the tolerances within these Marshall amps from 1967 - 1989.

I laugh nearly each time I open an old Marshall as the AC in & PT wiring is one of a few ways often using far too much wire. Needs to be nice & tidy.