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bigparsnip 5th June 2004 11:20 AM

Building a fender twin reverb in the UK
Well, I was in the pub last night and after a few beers the fact that I have been building amps cam up in conversation. a#t this point one of my friends who plays guitar said, can you build me a new guitar amp. Of course I said yes, did he have one in mind.

This was obviously a silly question, as he just grinned and said a fender twin reverb. So I have had a look about and found some schmatics for the version that I think I will use for this (the AA763) and now I need to find where to get some of the parts from and a littel advice on putting things together (I haven't done a vale amp before).

The tubes that are listed in the schematic are four 6L6GC output tubes, two 12AT7' in the preamp and reverb circuits, a 12AX7 and three 7025's for the rest. So, if anyone knows where I could pick up some suitable replacements in the UK to uses in place of these tubes (or anywhere which still has the originals?) that would be a very useful start.

The second thing I will probably need help with for this is transformers, both for the power supply (TR1) and output (TR3), as well as the two witch are sued inside the circuit (TR2 and TR4).

The link to the schematis is here and I realy would appreciate it if someone could point me in the right direction as to where I could find suitable transformers and tubes for this project.


benny 5th June 2004 12:08 PM

those should help you with trying to figure out which tranny's/chokes you will need as far as ratings go.

as i'm aussie, i have no idea where to get tubes and trannys from in the UK... but im sure there should be plenty...

you could also try looking in a place like ebay for original fender trannys. probably heaps cheaper too.

good luck with the amp... and make sure your friend understands before he starts paying for all this stuff, that this amp, while it will be close, isn't going to sound exactly like a fender twin reverb, as there are many things where your make will deviate from the original that will alter the sound... like the components used and what they're made of... the trannys you use, and even more so, the speakers & cabinet used. this will play a big part in the final sound... there are plenty others too... so tell him don't expect a clone, just a very nice amp that will be similar to a twin reverb.


bigparsnip 5th June 2004 12:18 PM

Hi, thanks for that link, ver useful, although at $220 dollars plus shipping to the UK for all the transformers I would need it's probably a little too pricey for what we wanted.

On a pluss side, I did find a scource for all of the tubes I will need for the project in the uk ( althugh these will also work out at about 100 for the lot, so perhapse the price for the transformers isn't too bad.

benny 5th June 2004 12:43 PM

welcome to the world of vacuum tubes buddy! one lesson you will learn very quickly, is that valves and trannies are not cheap! trust me... i'm just a poor teenager who has to scrimp and save for everything i buy!...
but anyway, did you get the ratings of the trannies you need??? that's why i really gave you those links... not to buy off him. if you want cheaper iron, you might get lucky and find what you want of ebay, or somewhere else seccond hand.. if not, just buy something that is what you need... so for example, if you need a 2k primary, 100W tranny... any tranny like this will do the trick. don't have to be a fender original/replica... like for example, hammond have a 1.9k primary, 120W tranny... so if you needed the 2k 100W one, this hammond tranny would do the trick.

let your friend know that he won't get it extremely cheap unless you can source seccond hand trannies and what not... i don't recomend seccond hand tubes though...
on the plus side, as a rough estimate, i could build a twin reverb for somewhere around $1500 aussie out of all new parts, including a set of speakers, and some wood and tolex for a cabinet. but to buy a genuine fender one, i would pay more than double this...

by the way, if this is your first tube amp, just be aware of the high voltages... :att'n: :hot: they're a bit nastier than the lower voltages found inside solid state amps.


bigparsnip 5th June 2004 12:48 PM

Hi, that's the unfortunate thing here, as the specs for the fender trannies don't seem to be mentioned, just the compatable part numbers. For example, I have found a transformer for less than $60 (and it's in a locak shop here in the UK :) ) but it's rated at 350-0-350V and 250mA and I'm not sure that the current rating will be high enough for the amp.

As far as the others go though, I haven't much idea what the specs should be which is my main problem at the momnet.

Oh, and I have been having a look about on ebay at the tubes too, and it looks like the 100 for the lot from the place above should be lowered quite a bit, which is nice.

benny 5th June 2004 01:36 PM


but it's rated at 350-0-350V and 250mA and I'm not sure that the current rating will be high enough for the amp.
The easiest way to know wether your tranny is up to the task is to get data sheets for all the tubes that will be in your amp. and then add up for all the max plate and screen dissipation values. just remember with something like a 12AX7 or any other dual triode that may be used, the ratings are given for one section in the tube... so you need to add up for every section, not just each tube... and remember to add on screen dissiaption for your pentodes as well as the plate dissipation.

example would be

plate dissipation: 24 watts max
screen dissipation: 3.5 watts max

plate dissipation: 1 watt max

so as you said, you have 4 x 6L6, so total dissipation of these would be 4x(24+3.5)=110W
and for example, say you use 2 12AX7's, using all sections, you have 4x(1)=4W

add these together... 114W

of course, it will vary... but as long as your tranny could handle 114W, you should be right for a guitar amp... actually, it's nice to have a tranny that's a little underated for a guitar amp, as it adds a bit of sag to the circuit. if it was a hifi amp, i'd tell you to over-rate the tranny, as sag isn't something you want in a hifi.

the 350-0-350@250mA tranny would be over rated for this, so you could use it if the voltage is right (if that is the tubes you are using.)... actually, this tranny should be good if the voltage is also right, as you say you have four 6L6's, and i can't see why there would be enough preamp tubes to push current draw up enough to outdo this tranny.

you will also need to make sure your filament suply can handle as well... to check this, all you need do is add up the current draw of the filaments on each tube, and check the filament winding can give this current. only difference here is that a dual triodes filament current draw is rated for whole tube, so only need to add once per tube instead of twice.

yes, ebay is good place to get tubes too... i have never bought tubes off it, but i sometimes see some real bargins on new tubes that people don't want anymore. if you're persistent enough, you might even save a huge amount of money by buying seccond hand/unwanted stuff... you just have to look hard and long enough.


benny 5th June 2004 01:41 PM

oh, by the way... i am guessing at the 2k primary on the OT based on the 4 x 6L6 configuration @ 100W... as there is a 2k 100W tranny listed on one of the pages i gave you... and 4 6L6's giving 100W would be running somewhere around class AB2, with a P-P imp of around that 2k mark.


bigparsnip 5th June 2004 01:51 PM

Thanks again for that, I guess I can go head with the local power trannie now then as it looks like it should be up to the job (I think the heater supply is rated at 3.5 A at 6.3V, although, it may be 7 depending on how they have defined the max current). I guess I will need to have a look about for some data sheets on the tuebes to see if this is ok though.

Thanks for the info on the output transformer too, I take it that if I do find one with a 2k input impedance, I should be looking at trying to get one rated at around 100W or so to cope here(, but it will have to have the split input primary and a single output right?

Also, if you want to give the tubes off ebay a go, have a look at this guy's store as he has lodes of russian valves and other bits and pieces at some veery good prices.

benny 5th June 2004 02:15 PM

you should find all the data sheets you need there. you can also download a program he has on his site, which has a whole lot of basic tube data in it for most tubes, and links to full on data sheets on the net.

without actually checking with the data sheets, i have a feeling you may need more than 3.5A heater current. you may want a sperate filament tranny if so, but this one won't cost you a lot seeing as you'll only need a small one as you've already got 3.5A on the filmanent winding on the main tranny... just use the extra one to make up the difference.

i might go to that store there in the future when i got more need for tubes... as for now, i don't see why i am going to need something like 4 rectifier tubes at once... i am a high school student who scrimps and saves to get only what i need at the time... i can't really be bothered thinking for the future needs. but thanks for the links anyway.


benny 5th June 2004 05:54 PM


but it will have to have the split input primary and a single output right?
sorry, completly missed this one...

yes, you're right...

there are two basic configurations for a power amp... that is single ended (SE) and push pull (PP). in your case, you have PP, so need the center tapped primary, or (obviously) a push-pull output tranny.

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