Marshall 100 PT with Marshall 50 OT, question please.

[BStringThumper]

Hi everyone,

Question, please.

I have a Marshall 2204 that needs a power transformer. I also have a Classic Tone 40-18053 100W power transformer. Is there any reason
I couldn't use this???? I know it's overkill for a 50W amp but it's what I got. I noticed a couple differences from the transformer information.
The 100W has a dedicated bias winding, which I probably won't use. The biggest difference I found on the 100W PT was the yellow center tap wire of the HT winding goes to the 50uf capacitors. But on the 2204 it goes to ground. On the spec sheet for the 100PT it shows a diagram of it going to capacitors and says under that "if only one capacitor is used, do not use the yellow wire, insulate it and do not connect to anything". I'm not using one or two capacitors it's going straight to ground.
Here is a link to the 40-18053 http://www.classictone.net/40-18053.pdf

I'm confusing myself thinking about this and need some professional advice.

Thank you very much.

[cerrem]

It's possible to run this PT in your 50W ...however there will be issues to deal with...
The 100W PT will be under loaded.... therefore voltages will be higher than expected in a 100W amp...
For example your heater voltage will likely be around 6.7V ...not a big deal...
Your DC B+ will be around 485 ...give or take when loaded.....
Make sure your filter caps are up to the task...
Try using the AC winding on the Stand-By switch...else if you use DC on the Stand-By switch ..your un-loaded voltages may exceed the 500V cap voltages...so wire them in series...
On a 100W circuit...the HV windings are configured in Voltage Doubler arrangement...
The 50W amp you wont use the YELLOW wire when the bridge is grounded in the tail...

[BStringThumper]

Hi Cerrem, thank you for the detailed reply. I already replaced the filter capacitors with F&T 50/50-500V caps. Now I'm wishing I'd gone higher on the voltage rating.
Great thought about the AC winding to standby switch.
OK, so I won't be using the HT yellow center tap. I'll insulate it. Thank you.

[pdf64]

For the cost of transformers these days, i would just get the correct set of iron for the amp you want.
A 100 watt PT seems likely to give grief down the road, by running the power tubes too hard.
But if you’re intent on this, a pair of 6550 / KT88 may be better suited to the likely conditions than EL34.

[BStringThumper]

Hi pdf64,
I had this 100W PT kicking around, just figured I'd use it.
I didn't think there was much of a voltage difference between the 50W & 100W? 440VDC & 485VDC?
Current draw will be 3.0 amps less (EL34 1.5A current draw x 2) The PT won't be working hard. The OT on the other hand with the 6550 tubes will be working hard. Thanks for the 6550/KT88 suggestion. I'm also going to look for a rugged 6L6 type tube comparable to a 6550?
Thank you both for your input.

[cerrem]

As already stated.... your main issue is the filter caps....the tubes you use should not be the problem....
STAND-BY switches do have a purpose in this situation...in as much as I am going to get a forum beating for saying that....
With no standby-switch the tubes are cold and NOT conducting current .....and with solid state diodes, your B+ will be really high above 500V until the tubes conduct and current flows to bring the B+ down ....
The cathode to heater voltages on the V2 12AX7 may also be above 200V at this cold start-up and cause a flash-over..
If you play your cards right.....you can get this work...

[g1]

Less current draw on the 100W PT means the voltages will rise above what they would be in a 100W.
That is where the concern lies.

[BStringThumper]

Hi,
I wired the HT AC winding to the standby switch first and then from the top of the standby switch to the rectifier.
It looks like from what everyone is saying is the voltages will be too high for the standard 500V caps regardless of what I do because I'm missing two more
output tubes to draw down the voltage as soon as the standby switch is flipped on... voltage flowing. Even if I had higher voltage rated capacitors how would I
avoid hitting the V2 12AX7 with 200+ voltage? I need to study a 100W schematic. Thanks.

[cerrem]

Hi,
I wired the HT AC winding to the standby switch first and then from the top of the standby switch to the rectifier.
It looks like from what everyone is saying is the voltages will be too high for the standard 500V caps regardless of what I do because I'm missing two more
output tubes to draw down the voltage as soon as the standby switch is flipped on... voltage flowing. Even if I had higher voltage rated capacitors how would I
avoid hitting the V2 12AX7 with 200+ voltage? I need to study a 100W schematic. Thanks.

Try adding a rectifier tube.... GZ34 ...
If you warm up the tubes before hitting STAND-By you should be OK....
Make sure your not biased too cold...

[pdf64]

...I didn't think there was much of a voltage difference between the 50W & 100W? 440VDC & 485VDC?
Current draw will be 3.0 amps less (EL34 1.5A current draw x 2) The PT won't be working hard. The OT on the other hand with the 6550 tubes will be working hard. Thanks for the 6550/KT88 suggestion. I'm also going to look for a rugged 6L6 type tube comparable to a 6550?
Thank you both for your input.

The secondary voltages noted as usually provided at a particular secondary current; maybe a typical idle current, maybe max rated current.
As the secondary current draw increases, the voltage decreases.
The % difference in secondary voltage between no and full load may be referred to as its regulation, tends to be in the range 5 -15%, with guitar amps at the higher end.
PTs intended for tube rectifiers tend to include a substantial degree of the necessary series plate resistance within their HT winding, the flipside of which is that they have poor regulation intentionally designed into them.

...The OT on the other hand with the 6550 tubes will be working hard...

I don't see why that should be so; please could you explain your reasoning for thinking this?

...I wired the HT AC winding to the standby switch first and then from the top of the standby switch to the rectifier...

Try adding a rectifier tube.... GZ34 ...
If you warm up the tubes before hitting STAND-By you should be OK...

That standby arrangement would 'hot switch' the reservoir cap on to the (tube) rectifier, which would seem to be inviting damagingly excessive surge (rectifier) plate current every time the amp was flipped into 'operate' mode, possibly leading to a short operational life ending with a lightning storm?
The typical Fender BF standby arrangement avoids such hot switching, eg https://el34world.com/charts/Schemat...b763_schem.pdf

[BStringThumper]

Then it would be like a JTM45. That's an interesting idea for adding some additional current draw, thanks.
Cerrem, can you please clarify for me what you mean by "warm up the tubes"? The
tube heaters will be heating when I turn on the main power switch. I always allow the tubes time to warm up
before flipping the standby switch. The HT hits the tubes when the standby switch is flipped, regardless of tube or
solid state rectified.... right? Am I missing something??

[Helmholtz]

Then it would be like a JTM45. That's an interesting idea for adding some additional current draw, thanks.
Cerrem, can you please clarify for me what you mean by "warm up the tubes"? The
tube heaters will be heating when I turn on the main power switch. I always allow the tubes time to warm up
before flipping the standby switch. The HT hits the tubes when the standby switch is flipped, regardless of tube or
solid state rectified.... right? Am I missing something??

Does your 100W PT provide a 5V heater supply for the rectifier tube?

[BStringThumper]

No it doesn't have the 5V winding. Good point.
I was hoping to hear if the higher plate voltage made a tonal difference and also not stressing
the power transformer in any way. Plus, as I said, I already had this 100W PT.
It's looking more and more like I should buy the correct transformer.
Thanks.

[J M Fahey]

I canīt imagine a **100W** Marshall using tube rectifiers, so why even ask about a 5V winding in a relatively modern PT?

[Helmholtz]

I canīt imagine a **100W** Marshall using tube rectifiers, so why even ask about a 5V winding in a relatively modern PT?

I know, but the idea was to use a GZ34 to reduce voltages