PT question : 425-0-425 from this vintage iron

[rogb]

I have been testing the old 50w PT I am going to use for my next build.

So far I have 425 v AC on each B+

6.3v AC from the rectifier tap

6.9v AC from the heaters

53v AC(was unused as amp was cathode biased)

21v AC (was unused).

This transformer (un branded ) ran 2x EL34, I x GZ34, 2xEF86 and 1xECC83.

Am I correct in saying this will give me ~ 600vDC on the B+.
Could be a bit high for a #102?
I could run a zener on the CT I guess if needed

[talbany]

Like 100v too high!
5U4 rectifier will drop about 30v plus the load around 15v would put it around 555V under load depending on bias setting.. 53v is a bit low for a bias tap unless you set it up for a voltage doubler (I would do) or do a parasitic style..
I am not too sure if I would try to drop 100v through the zeners..I would recommend getting a new transformer running at 450V and save that one for an EL-34 amp..

Good Luck..

Tony

[rogb]

Like 100v too high!
5U4 rectifier will drop about 30v plus the load around 15v would put it around 555V under load depending on bias setting.. 53v is a bit low for a bias tap unless you set it up for a voltage doubler (I would do) or do a parasitic style..
I am not too sure if I would try to drop 100v through the zeners..I would recommend getting a new transformer running at 450V and save that one for an EL-34 amp..

Good Luck..

Tony

Hi Tony. yeah 5U4 is what I use on my AB763 for a real voltage drop difference between tube and SS rectifier.

I guess 425 0 425 will never be right, looks like I need a 345 0 345.
I didnt realise until today that the old lump kicked out that many volts, it is a big old lump

[talbany]

Like 100v too high!
5U4 rectifier will drop about 30v plus the load around 15v would put it around 555V under load depending on bias setting.. 53v is a bit low for a bias tap unless you set it up for a voltage doubler (I would do) or do a parasitic style..
I am not too sure if I would try to drop 100v through the zeners..I would recommend getting a new transformer running at 450V and save that one for an EL-34 amp..

Good Luck..

Tony

Hi Tony. yeah 5U4 is what I use on my AB763 for a real voltage drop difference between tube and SS rectifier.

I guess 425 0 425 will never be right, looks like I need a 345 0 345.
I didnt realize until today that the old lump kicked out that many volts, it is a big old lump

Yeah a bunch of amp companies in 70's and 80's were going more for volume and headroom so the voltages climbed and got pretty sterile..That's a strange one that it has a rectifier tap though, (Ampeg did a few).If you want the feel of the original 102 you don't want a rectifier tube..Look for something FWB that puts it at 450V or slightly lower!

Again..Good Luck!!

T

[rogb]

This is what it came from


How does a Mojo Bassman sound to you?

I will try to resist MM and the big bucks, but I had a particular MM OT in mind, may just go for broke, suck up the hype and MM it all the way

Shame really I have had this trafo since 75 and I really wanted it in my amp as a link to the old days...

[talbany]

The Mojo/Heybor's are fine..If you decide not to spend the cash and go MM look at a set from Magnetic Componets (Bassman Replacements) you could get both Pwr and OPT shipped in the same box and maybe save youself some bucks that way..Mag Componets has really stepped up there game in the last few years and are also wonderful transformers for the $..

I have to get over to the UK.. High on my bucket list..Sold Many amps there!..

All The Best!

Tony

[Roe]

to reduce the b+, connect the 345s to rectifier, then to 300mA choke (w/o cap) then to mains cap

[rogb]

to reduce the b+, connect the 345s to rectifier, then to 300mA choke (w/o cap) then to mains cap

Thanks Roe, I have 850vAC, I need 690vAC, say 490-500vDC after rectification - I don't think I can sensibly reduce 100v + as Tony says.

I am looking at some Bassman iron now and put this old PT away for a Marshall build or something

[Roe]

to reduce the b+, connect the 345s to rectifier, then to 300mA choke (w/o cap) then to mains cap

Thanks Roe, I have 850vAC, I need 690vAC, say 490-500vDC after rectification - I don't think I can sensibly reduce 100v + as Tony says.

I am looking at some Bassman iron now and put this old PT away for a Marshall build or something

the choke method I sketched above should give you ca 400v. Seehttp://www.aikenamps.com/Chokes.html

[rogb]

to reduce the b+, connect the 345s to rectifier, then to 300mA choke (w/o cap) then to mains cap

Thanks Roe, I have 850vAC, I need 690vAC, say 490-500vDC after rectification - I don't think I can sensibly reduce 100v + as Tony says.

I am looking at some Bassman iron now and put this old PT away for a Marshall build or something

the choke method I sketched above should give you ca 400v. Seehttp://www.aikenamps.com/Chokes.html

OK, interesting.
Reading Aiken, it seems for a 50w amp. you need a 250mA 100-200ohm 5-20 Henry choke that will handle the max output current.. It would be good to give it a try if such a choke can be found.
The only problem is that deviating too much from #102 spec will alter the tonality of the amp.
Hey it might make it better though



EDIT: Just found that a 5H 300mA choke weighs in at about 5.5lbs.
Don't think I have the space for it on my chassis . It also costs not much less than a new PT - oh well, good plan anyway, thanks.

[Roe]

...it seems for a 50w amp. you need a 250mA 100-200ohm 5-20 Henry choke that will handle the max output current.. It would be good to give it a try if such a choke can be found.
The only problem is that deviating too much from #102 spec will alter the tonality of the amp.
Hey it might make it better though



EDIT: Just found that a 5H 300mA choke weighs in at about 5.5lbs.
Don't think I have the space for it on my chassis . It also costs not much less than a new PT - oh well, good plan anyway, thanks.

I recommend the hammond chokes (193 series probably): either 2.6H 300mA or 5h 300mA. the former isn't big and heavy but the latter is. they cost around 40$

Link: http://www.hammondmfg.com/193.htm

[rogb]

Roe, I'm going to order a full Bassman set from MC, I've been swayed away from MM, so I can get the 3 trafo parts shipped to the Uk for about the same as a MM OT!
I'd like to get this amp right and supplying the right voltage seems a good place to start.
I really appreciate your advice and help

[Roe]

the classictone iron sounds good to me