WHo is the tube type technology guru here ?

[dacflyer]

i have a 100 watt guitar amp that uses 4) EL34 tubes and 1) 12AT7
the transformer is shorted.. the voltages are 360-0-360 and 3.2-0-3.2
i have been looking online for replacement transformers, but i do not know how many Ma. i should be looking for for the
B+ HV section and filament amps.
i found a transformer that claimed to output 120 watts, but not sure if this is good for 120 watt output of if it uses 120watts of power.
does anyone have any info on how much power is needed for the tubes i listed ?

[CeaSaR]

Who is the manufacturer? If they are still in business, maybe you can find a certified repair shop that can order the transformer for you. That's what my son did when his Peavey Classic 30 toasted its power windings. The guy we went to said he normally doesn't do that, but when he found out I was a long time hobbiest, he felt comfortable doing so.

Hey, if I can find his info, I'll pm you.

CeaSaR

[CeaSaR]

Pm sent.

CeaSaR

[haklesup]

transformers are generally efficient so ignoring a small inefficiency, for the most part input power is equal to output power. However if the amp is rated for 120W, the transformer will need to noticeably more than that because there are lots of inefficiencies in the signal path. So knowing the power in W (should be VA) and the voltage you can determine the current. By observing the wire gauge, you can roughly check that.

Transformer design has hardly changed since that amp was made. If the replacement is about the same size, odds are its the right power.

http://www.audiomatica.com/tubes/el34.htm
http://www.drtube.com/datasheets/12at7-rca1954.pdf

5 tubes are not that many, odds are the filament winding at 6.4V will be sufficient, TV sets of the day had over a dozen tubes

[dacflyer]

the amp is made by Carvin VTR2800

http://www.carvinservice.com/crg/manuals/vtr-2800.pdf

the amp is 100W, the transformer i was looking at is rated for 120W , but the B+ ma. seemed low to me
how can ma. equate to 100watts. is that a secret of tube amps ? i know tubes are voltage devices where as transistors are current devices. and i was not sure if the transformer provided enough tube current. (ahhh,,maybe the power to drive the 100W comes from tube current ? ) i wish i knew more about tube technology.

[sghioto]

Power = Voltage * Current, whether it's tubes or transistors. If the transformer is rated at 120 watts and the voltage is 360 volts then the rated current is 120/360 or 333ma. The replacement transformer sounds about right as hakelsup mentioned. A 100 watt amp probably consumes around 120 watts at full output. The manual didn't list in the specs about the actual power required, but it might be stamped on the plate usually found on the back of the chassis somewhere.

Steve G

[dyarker]

Your amplifier is mixed technology. Vacuum tube, transistor and integrated circuit. There are at least two large transformers. An audio output transformer to couple high impedance vacuum tube out to low impedance speakers; and a power supply transformer for B+ and filiments. Power for semiconductor sections may be from a tap on the supply transformer, or a third transformer.

The 12AT7 has 12.6V filiment, but is filiment is center-tapped so the tube can be run on 6.3V by connecting the ends together. All five tubes consume 40W of 6.3V (4 times 1.5A, plus 300mA for the 12AT7).

Considering the relative inefficiency of linear amplifiers, I expect the power transformer to be around 300W, or more.

(saved my work, and saw someelse answered mA to W.)

?? Which winding is shorted? B+, or heater, or speaker?

[sghioto]

dacflyer can you send us a link to the replacement transformer you're talking about?

Steve G

[dacflyer]

the B+ winding is shorted, you can hear is sizzling inside of the transformer.

as for the transformer i was referring to, i cannot find it online now Grrrr. i'll keep looking , then let you know.. i know it was a torroid type transformer.

i did find this one, i almost think this will work, looks beefy.. and the voltages i need.
i'll have to combine leads to make
360-0-360 and parallel the 6.3 windings (probably don't even need to parallel them)

http://www.antekinc.com/as-4t360-400va- ... ansformer/

[sghioto]

That one from Antek should fill the bill at a nice price as well. You'll need more then 4 amps just on the EL34s. Nominal heater current is 1.6 amps. Either parallel the windings or split the load between the two windings.

Steve G

[dyarker]

Yep! But don't forget to supply voltage for semiconductor sections.

Cheers,

[sghioto]

Yep! But don't forget to supply voltage for semiconductor sections

Oops! You could use a second transformer for the semiconductors. If I'm looking at the correct schematic a 52 volt ac winding is needed. The Antek AN-0124 unit with the windings in series should work and it's only ten dollars.

Steve G

[dacflyer]

oh man, i totally missed seeing the 52 volt winding.. Grrrrr
time to search more.. i do not wanna have to add another transformer.

this is the closest i have found, i wonder if it is close enough to use..
http://www.ebay.com/itm/upgrade-2x-120V ... 2a3c039050

[sghioto]

If you can operate the 12AT7 heater on the 5 volt winding it will work. The 3.2 volt secondaries are rated at 6 amps total and that will just supply the four EL34 tubes. I would still go with the two Antek transformers. What's the big deal?

Steve G

[dacflyer]

trying to just use 1 transformer,, 2 will be a last resort..