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Vintage threads from the first ten years

Damn, not enough B+

3/4/1998 8:39 PM
Darren	Damn, not enough B+ Well the "sort-of-pretend" 50W marshall i have built (from the circuit here) has run into an obsticle that maybe one of you guys can assist me with. Too low b+...The power transformer was pinched from an old Valve TV, and with full wave SS rectification using 2x330uF 400V caps as filters i get 288VDC on the plates of my 6L6's, which as i'm sure you have guessed, delivers about as much power as my walkman.....well not exactly, but not quite the kinda power i was after. Now i tried wiring the power supply up in a voltage doubler configuration and wound up the mains (240V here btw) on my variac. well i got to around 70VAC in and allready i was 400VDC out under no load, so i guess this isn't going to work either, unless of course i replace the 140ohm 15W resisitor i'm using instead of a choke with a very much higher value... Any suggestions other than get a different PT? Thanks again Darren

3/4/1998 8:39 PM

Darren

Damn, not enough B+

Well the "sort-of-pretend" 50W marshall i have built (from the circuit here) has run into an obsticle that maybe one of you guys can assist me with. Too low b+...The power transformer was pinched from an old Valve TV, and with full wave SS rectification using 2x330uF 400V caps as filters i get 288VDC on the plates of my 6L6's, which as i'm sure you have guessed, delivers about as much power as my walkman.....well not exactly, but not quite the kinda power i was after. Now i tried wiring the power supply up in a voltage doubler configuration and wound up the mains (240V here btw) on my variac. well i got to around 70VAC in and allready i was 400VDC out under no load, so i guess this isn't going to work either, unless of course i replace the 140ohm 15W resisitor i'm using instead of a choke with a very much higher value...
Any suggestions other than get a different PT?

Thanks again
Darren

3/5/1998 5:52 AM
Pete	Rather than go for a 50w Marshall clone, change your power tubes to 6V6's and go for a Deluxe. Pete

3/5/1998 1:16 PM
Stephen	Darren, I don't have time to work out the on-load voltages for this - it's probably quicker to try it and measure....I know it's not the proper kind of PS regulation for a guitar amp, but I think you could do a Choke-input voltage doubler PS, which should give you a useable B+ for a 50W amp. Depends on how big your present choke is. You'll have the 6L6 plate currents going through the choke as well, so it will need 150ma=plus continuous rating. And you'll may need a bleeder resistor from B+ to 0 V, if the amp's idle current isn't enough to pull the choke-input PS down into its (nearly) constant voltage region. This minimum current value depends on the inductance of the choke - although a 'swinging' choke may make things a bit simpler. All the tech for this is in RDH4. There are basic choke-input graphs in many tube data books, but not usally for swinging chokes, or voltage doublers. You'll have to try what you've got and see what it measures. You could probably use a regular small-current (screen and preamp) choke to see if you can get a suitable B+ - just don't try running the amp at more than 1/4 power. If this works, you might need to buy a bigger choke, but these can sometimes be found at hamfests and surplus stores. ( Do you have hamfests in Oz? If there is one in NZ, I have never managed to trace it. I am very envious of these guys in the US who can just pick up an old transmitter chassis or Tectronics scope for a few dollars) Even though this is a choke-input PS, it would be wise to hang a small cap (say 0.5uF/1kV) from the _input_ side of the choke, to ground...or put a r-c snubber across the choke...or both, to minimise transient voltage spikes across the choke. There is also a kind of voltage doubler where the output voltage can be adjusted a bit up and down by varying the size of one electrolytic cap. It has the snag that both + and - of this cap are at a fairly high voltage above chassis, so you would have to put the cap in a reliable insulating sleeve, and where fingers can't touch the outer can. I'll dig out the diagram and post it if you want. These are just _some_ possibilities. I'm sure the other guys will have some other ideas as well. - Stephen

3/5/1998 1:16 PM

Stephen

Darren,
I don't have time to work out the on-load voltages for this - it's probably quicker to try it and measure....I know it's not the proper kind of PS regulation for a guitar amp, but I think you could do a Choke-input voltage doubler PS, which should give you a useable B+ for a 50W amp. Depends on how big your present choke is. You'll have the
6L6 plate currents going through the choke as well, so it will need 150ma=plus continuous rating. And you'll may need a bleeder resistor from B+ to 0 V, if the amp's idle current isn't enough to pull the choke-input PS down into its (nearly) constant voltage region. This minimum current value depends on the inductance of the choke - although a 'swinging' choke may make things a bit simpler. All the tech for this is in RDH4. There are basic choke-input graphs in many tube data books, but not usally for swinging chokes, or voltage doublers. You'll have to try what you've got and see what it measures. You could probably use a regular small-current (screen and preamp) choke to see if you can get a suitable B+ - just don't try running the amp at more than 1/4 power.
If this works, you might need to buy a bigger choke, but these can sometimes be found at hamfests and surplus stores.
( Do you have hamfests in Oz? If there is one in NZ, I have never managed to trace it. I am very envious of these guys in the US who can just pick up an old transmitter chassis or Tectronics scope
for a few dollars)
Even though this is a choke-input PS, it would be wise to hang a small cap (say 0.5uF/1kV) from the
_input_ side of the choke, to ground...or put a r-c snubber across the choke...or both, to minimise transient voltage spikes across the choke.
There is also a kind of voltage doubler where the output voltage can be adjusted a bit up and down by varying the size of one electrolytic cap.
It has the snag that both + and - of this cap are at a fairly high voltage above chassis, so you would have to put the cap in a reliable insulating sleeve, and where fingers can't touch the outer can. I'll dig out the diagram and post it if you want.
These are just _some_ possibilities. I'm sure the other guys will have some other ideas as well. - Stephen

3/5/1998 7:57 PM
Darren	Stephen, Thankyou for an obviously well thought out response, had i not discovered what i mention in my other post about power trannies, i would be testing some of you suggestions. However what i wanted to say is i don't know about hamfests over here (Brisbane BTW) i have been getting parts by chatting to other techs (i am in the electronic repair biz, just more computer and digital oriented than analouge and old audio area) and meeting guys who still have old "junk" stockpiled in their sheds/under the house etc. Usually a bottle of their preferred spirit and a nostalgic chat about the "good old days of tube type servicing" (of which i actually know bugger all 'cause i'm only 27) and i get free access to rummage through their old bits and pieces. Short of buying everything new, this is the only method i have for getting bits.......... Cheers Darren

3/5/1998 7:57 PM

Darren

Stephen,
Thankyou for an obviously well thought out response, had i not discovered what i mention in my other post about power trannies, i would be testing some of you suggestions. However what i wanted to say is i don't know about hamfests over here (Brisbane BTW) i have been getting parts by chatting to other techs (i am in the electronic repair biz, just more computer and digital oriented than analouge and old audio area) and meeting guys who still have old "junk" stockpiled in their sheds/under the house etc. Usually a bottle of their preferred spirit and a nostalgic chat about the "good old days of tube type servicing" (of which i actually know bugger all 'cause i'm only 27) and i get free access to rummage through their old bits and pieces. Short of buying everything new, this is the only method i have for getting bits..........

Cheers
Darren

3/6/1998 1:22 PM
stephen	Yep....much the same here. I buy my tubes in three currencies - US dollars, Kiwi dollars..and Steinies. For US sources of tubes and parts specific to guitar amps, I guess you know about Mojo Antique Electronic, and Triodeel.com. There are some Oz and NZ NOS tube sources ...but our US friends have so much choice already, I don't think we need to tell them about those....( anyway, not while we're still getting British GZ34's for $20 BTW... your ex-tv tranny would probably be perfect for a 2x6V6 amp - or depending on its current rating, maybe a 4x6V6 amp. Russian 6V6's are cheap, and there's a good chance they would be reliable on that B+, less a bit for cathode resistor drop.

3/6/1998 1:22 PM

stephen

Yep....much the same here.
I buy my tubes in three currencies -
US dollars, Kiwi dollars..and Steinies.
For US sources of tubes and parts specific to guitar amps, I guess you know about Mojo
Antique Electronic, and Triodeel.com.
There are some Oz and NZ NOS tube sources
...but our US friends have so much choice
already, I don't think we need to tell them
about those....( anyway, not while we're
still getting British GZ34's for $20 :-)

BTW... your ex-tv tranny would probably be
perfect for a 2x6V6 amp - or depending on its current rating, maybe a 4x6V6 amp. Russian
6V6's are cheap, and there's a good chance they would be reliable on that B+, less a bit for cathode resistor drop.

3/5/1998 7:46 PM
Darren	Okies, thanks for the suggestions, I have done a bit more research and discovered that here in Australia, i can get a brand new power transformer, wound to my specs ( in this case i asked for a 5 Amp 6.3VAC winding and another AC winding of sufficient size to provide me with 400VDC@up to 500mA after bridge rectification) for around $115 AUS..now i thought this seemed rather cheap, and it is a drop in replacement size-wise too. Anyone with experiance buying power trannies, is this a fair price, good price, or don't touch with a ten foot pole price. Any thoughts? By the way, from another company i can get a toroidal transformer wound to the same specs (which incedently is apparently around 200-250VA) for $85 Aus, but a toroid woudn't look quite right in this vintage chassis i feel, but it is worth considering for future ground up projects...... TGIF Darren

3/5/1998 7:46 PM

Darren

Okies, thanks for the suggestions,

I have done a bit more research and discovered that here in Australia, i can get a brand new power transformer, wound to my specs ( in this case i asked for a 5 Amp 6.3VAC winding and another AC winding of sufficient size to provide me with 400VDC@up to 500mA after bridge rectification) for around $115 AUS..now i thought this seemed rather cheap, and it is a drop in replacement size-wise too.
Anyone with experiance buying power trannies, is this a fair price, good price, or don't touch with a ten foot pole price. Any thoughts?

By the way, from another company i can get a toroidal transformer wound to the same specs (which incedently is apparently around 200-250VA) for $85 Aus, but a toroid woudn't look quite right in this vintage chassis i feel, but it is worth considering for future ground up projects......

TGIF
Darren

3/10/1998 10:19 AM
Stephen	Darren, Sorry about delay in replying to this...I was sidetracked by other things at this end. It makes a nice change to find items and services which cost _less_ down here than they do in the US. One-off transformers seems to be one of these. $100-150 is about what I pay for a typical tube amp PT, depending on rating, and how much copper and iron. If you plan to use your amp for hours at a time, then check that the offered tranny is rated for continuous use, not intermittent domestic use. Also transformer design includes a permitted temp rise above surrounding air. In a tube amp - and especially in a combo amp with the usual inverted chassis in the top of the cab- there may be little air-flow, and the air surrounding the PT may easily reach 30 degrees ( make that 50 degrees if you're playing a desert gig The design temp rise goes on top of that - It's quite easy to end uo with a PT which is running too hot to touch! This may be ok with modern tranny materials, but I have chosen to get my combo-amp PT's designed for a lower temp-rise in continuous use. I am sure tranny designers in Oz have learned to allow for the high ambient temp in some parts of the country, but maybe you should let them know if your PT will also be working in a limited air flow and/or in raised local air temperature. They might say that's ok, or they might suggest the next size up for a few $$ more. Toroids: I'm still a bit cautious about toroids for tube amp power supplies. There _are_ people who understand about high-voltage windings on toroids, but IMHO too many products rely too much on the integrity of the wire varnish insulation. I still like to see a physical barrier between windings. Your old (1989) standard for separation of windings on a toroid was C126 ( maybe changed now) and I don't know whether it also covered separation of HV and heater secondaries from each other, as well as from the input winding, You'd have to ask the people who quoted you. - Stephen

3/10/1998 10:19 AM

Stephen

Darren,
Sorry about delay in replying to this...I was sidetracked by other things at this end.
It makes a nice change to find items and services which cost _less_ down here than they do in the US. One-off transformers seems to be one of these. $100-150 is about what I pay for a typical tube amp PT, depending on rating, and how much copper and iron. If you plan to use your amp for hours at a time, then check that the offered tranny is rated for continuous use, not intermittent domestic use.
Also transformer design includes a permitted temp rise above surrounding air. In a tube amp - and especially in a combo amp with the usual inverted chassis in the top of the cab- there may be little air-flow, and the air surrounding the PT may easily reach 30 degrees ( make that 50 degrees if you're playing a desert gig :-)

The design temp rise goes on top of that - It's quite easy to end uo with a PT which is running too hot to touch! This may be ok with modern tranny materials, but I have chosen to get my combo-amp PT's designed for a lower temp-rise in continuous use.
I am sure tranny designers in Oz have learned to allow for the high ambient temp in some parts of the country, but maybe you should let them know if your PT will also be working in a limited air flow and/or in raised local air temperature. They might say that's ok, or they might suggest the next size up for a few $$ more.

Toroids: I'm still a bit cautious about toroids for tube amp power supplies. There _are_ people who understand about high-voltage windings on toroids, but IMHO too many products rely too much on the integrity of the wire varnish insulation.
I still like to see a physical barrier between windings. Your old (1989) standard for separation of windings on a toroid was C126 ( maybe changed now) and I don't know whether it also covered separation of HV and heater secondaries from each other, as well as from the input winding, You'd have to ask the people who quoted you.
- Stephen

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