50W ODS Clone - What output transformer do I want?

[RJ Guitars]

Hello All - I am putting together some parts and doing an inventory to build my first 50W ODS clone... I have a steel chassis 2x8x19 and a power transformer already mounted in it.

The power transformer I have makes 375 volts per side, will that work? It's a big old heavy unit that I've had on the shelf for 20 years... I think it's an old Bassman tranny, but I'm not certain.

FEB 3 update - I found a number on the power transformer - 22R07. This turns out to be a Thordarson 350-0-350 which is a bit more in line with what we were looking for. My actual measurements of 375-0-375 are likely due to the high grade of 110V AC we get here... I know I always see a higher than expected voltage coming off of power transformers here...

I don't have an output transformer, does anyone have a suggestion? I don't need a lot of wattage so two output tubes will be more than sufficient...

I bought an OPT from a friend - 40 watt very heavy piece of iron from Hammond - Model P-T1650H ...

Also in some of the chassis photos I see a third transformer - which I think is actually a choke mounted between the power and output transformers. Can anyone tell me more about this... is it just power supply filtering? If I read the schematic I think it says 300 to 500 ohms?

I have all the weight I can handle so I'll go for the resistor and forget the big choke...

I appreciate any assistance you have to offer...

rj

[Bob-I]

Hello All - I am putting together some parts and doing an inventory to build my first 50W ODS clone... I have a steel chassis 2x8x19 and a power transformer already mounted in it.

The power transformer I have makes 375 volts per side, will that work? It's a big old heavy unit that I've had on the shelf for 20 years... I think it's an old Bassman tranny, but I'm not certain.

That's a tad on the high side of normal. My PT's are all in the 300-320 per side range. I think you'll be ok with it.

I don't have an output transformer, does anyone have a suggestion? I don't need a lot of wattage so two output tubes will be more than sufficient...

If you plan to use 6L6's a Bassman replacement from AES or New Sensor will work great. For EL-34's a MArshall 50 watt replacement is a good choice.

Also in some of the chassis photos I see a third transformer - which I think is actually a choke mounted between the power and output transformers. Can anyone tell me more about this... is it just power supply filtering? If I read the schematic I think it says 300 to 500 ohms?

You can use either a choke OR a 300-500 ohm 10 watt resistor. Often the resistor is called the Robben Ford mod, I'm not sure why but I've done both. It's a slightly different feel at high volumes.

Go over the the files section and look through the layouts and BOM (Bill of Materials) for more information.

[tubetek]

Howdy RJ-
I'm assuming that you will be using 6L6 for the outputs...
If you want max clean headroom, you can do like the VibroVerb
clone builders and use a Fender TwinReverb OT. The 4 ohm to 2000 ohm
ratio converts to 8 ohm to 4000 ohm, a near perfect match for the 6L6 pair.
Hope this helps
tubetek

[RJ Guitars]

I've found a few more components and have the chassis almost finished... that is all the mounting holes done.

I located a pair of output trannies as a package deal and also had to buy a couple big chokes along with them... still a good good price for the package. Now I have a duplicate of all three pieces of iron... does a guy need two of these amps?

I have a good stock of 6L6, 5881, and EL34 tubes... I know that 6L6's seem to be the norm... is there any reason to that I should favor or stay away from the 5881s or the El34's?

[Sven]

Greetings,

What are the best impendances of the OT primary winding for the following output configurations in these amplifiers:

1. 2x 6L6

2. 2x EL34

3. 2x 6V6

with assumption that the load is one 8 Ohm speaker ?

Are there major variations with various models, such as Hybrid A (by the way, is Hybrid A actually the same amplifier as Non-HRM?), Non-HRM, HRM, and other models/schematics?

As I recall, usually Marshalls have 2x EL34 in 50Watt models, with 3,5k Ohm primary impendance into 8 Ohm speaker load. Is it true that 6L6 would be 4k Ohm? Anyway, if someone wishes to comment, perhaps it would clarify some issues. Thank you.

Sven

[Bob-I]

Greetings,

What are the best impendances of the OT primary winding for the following output configurations in these amplifiers:

1. 2x 6L6

2. 2x EL34

3. 2x 6V6

with assumption that the load is one 8 Ohm speaker ?

Are there major variations with various models, such as Hybrid A (by the way, is Hybrid A actually the same amplifier as Non-HRM?), Non-HRM, HRM, and other models/schematics?

As I recall, usually Marshalls have 2x EL34 in 50Watt models, with 3,5k Ohm primary impendance into 8 Ohm speaker load. Is it true that 6L6 would be 4k Ohm? Anyway, if someone wishes to comment, perhaps it would clarify some issues. Thank you.

Sven

You should remember from AC theory that transformers don't actually have impedance, they reflect impedance based on the turns ratio and core.

That said, 2x6L6's like to see about 5K, 2x EL-34's 2.5K and 6V6's about 8K.

As for the Hybrid-A? Dumble used Fender values for the preamp tubes in the 70's amps, then changed these values later when he used the HRM. The Hybrid-A is a 70's amp using later HRM values for plate load and cathodes.

[RJ Guitars]

My OPT has a 6.6K impedance with a 4,8, and 16 ohm tap and is suggested for 6V6's, 6L6's, and El-34's... It is rated for 40 watts. I have a good stock of tubes so I expect to try the various options until something sounds really good.

I actually have a regular gig (church) where I just can't play very loud so I don't expect to use the wattage, but I'd really like to get the sound of a Dumble at lower volumes...

Also, I'd like the clarification on the Hybrid A / HRM differences and any of the other subtlties. I've collected up several schematics... the Hybrid A has values for a 97 model and a 77 model... are there significant differences in the sound they put out?

I plan to make my amp using Normsters D-Lite layout...what model is that?

And finally... on the schematic and layout I see some of the pots are listed as 250KA or 250KB etc... Does this mean A for audio taper and B for Linear or did I miss the boat altogether?

rj

[Sven]

You should remember from AC theory that transformers don't actually have impedance, they reflect impedance based on the turns ratio and core.

Greetings,

Yes, of course, we often simplify the language in communications. Bob-I is right, certainly.

So, what do you think about Fender Bassman OT, for 2x 6L6. I think they are 4k Ohm, with 2, 4, 8 Ohm taps?

And, while on the subject of transformers, let me make an offer to this excellent forum, in the spirit of courtesy and sharing ...

I have an excellent MS Excell spreadsheet, translated from Russian into English, for those participants who would perhaps ¨like to roll their own¨ Power and Output Transformers (Push-Pull and Single-Ended) for valve amplifiers. The spreadsheet is amazing ... one can introduce several input parameters and design along the lines of included (embeded) mathematical formulas (simple math). Source of formulas is both American (old Electro-Voice book, or something like that?) and Russian, as compiled in a paper by a Russian valve amplifier afficionado Mr. Vassilchenko and than used to put together this excellent MS spreadsheet.

Input parameters are:

1. Amplifier output power (Watt)
2. Source internal resistance (i.e. tube's internal resistance) (Ohm)
3. Transformer's input resistance which is the anode load for the tube (Ohm)
4. Load impedance (loudspeaker's impedance) (Ohm)
5. Lowest frequency to be passed @ -1 dB (Hz)
6. Highest frequency to be passed @ -3 dB (Hz)

One could choose various core sizes, and one immediately sees what it does to the rest of the calculations.

Results are very informative, both theoretically and practically, featuring wire diameters for primary and secondary, total number of windings, number of windings per layer, and so on and so forth.

If someone is interested in making own ¨custom¨ Power and Output transformers, or learning something more about Transformers in general and in practical terms, just send PM with regular e-mail (I am not certain about sending of attachments via these forums, yet), and I shall send that spreadsheet as attachment.

It would be very interesting if some participant, very knowledgeable about this valve amp actually put together a ¨D'Iron¨ OT for this type of amp, for example.

All the best,

Sven