I have just completer a spitfire build
http://www.diyguitarist.com/Schematics/SpitfireSchematic.bmp
I used an old output transformer from a teisco AMP, but it is Way to small, and overall the AMP sunds thin and Weak...
Just ten kilometers from my house, a guy is selling a mercury magnetics OT.
It is made for a delux reverb, so the primary is 8k, and I believe the spitfire uses a OT with a much lower primary impedance. 6k or sommerens like that
I am sure it Will work, but curious to hear if there are anyone out there, Who believe that the original/untraditional OT in a spitfire is important in a build like this?
http://www.diyguitarist.com/Schematics/SpitfireSchematic.bmp
I used an old output transformer from a teisco AMP, but it is Way to small, and overall the AMP sunds thin and Weak...
Just ten kilometers from my house, a guy is selling a mercury magnetics OT.
It is made for a delux reverb, so the primary is 8k, and I believe the spitfire uses a OT with a much lower primary impedance. 6k or sommerens like that
I am sure it Will work, but curious to hear if there are anyone out there, Who believe that the original/untraditional OT in a spitfire is important in a build like this?
Oh, obviously this transformer is interesting because I Can get it cheap, and save the expensive shipping from abroad
The spitfire output is designed around a 4 K primary. But having said that, the Mercury Mag transformer is going to be quite/very good. If it has multiple secondaries, you can ''mismatch'' the speaker impedance to reflect back the load to get about 4K primary.Hope this makes sense: Connect an 8 ohm speaker into the 4 ohm secondary tap. You reflect back to primary at 4K instead of 8 K. That should get you to an optimal impedance. If you have a 4 ohm speaker that may drive it a tad hard. 16 ohm cab can plug into 8 ohm tap.
PS: Printer2 is right that EL 84s like the 8K. Even if you match the speaker taps, the Mercury OT will sound very good and you will only get a slightly lower volume. It will not blow anything up. The deluxe reverb OT I believe is rated much higher voltage etc than the spitfire.
Thanks for the replies...
it only has an 8 ohm tap on the secondary....But I´ve got both 8 ohm and 16 ohm speakers that I want to try out.
I´ll install it over the weekend, and let you know what sounds the best....
it only has an 8 ohm tap on the secondary....But I´ve got both 8 ohm and 16 ohm speakers that I want to try out.
I´ll install it over the weekend, and let you know what sounds the best....
Hooked up the output trandsormer tonight, with an 8ohm speaker.
The first impression is a much better sound and way more bass! But the volume is not impressive? And there is no gain/distortion what so ever. Volume almost fades out when toneknob is turned towards the bass side?
The voltages seem fine:
Powertubes:
pins 7: 351 vdc
pins 9: 352 vdc
V1
pins 1,6: 150 vdc
pins 3,8: 1,25 vdc
V2
pins 1,6: 253,258 vdc (should they be exactly the same?)
pins 3,8 : 66 vdc
B+ is 356 vdc
1st 33uf cap 356 vdc
2nd 33uf cap 354 vdc
1st 22 uf cap 335 vdc
2nd 22 uf cap 323 vdc
heater voltage (read between pins 4,5 - 9) is 6,8 vac ( a tad high?)
Cathode resistor is 150 ohms, and the voltage across the resistor is 13,2 vdc
Plate voltage pins 7-3 reads 338 vdc
Using the weber bias calculator gives 14w/41ma per tube
i know this is hot, but shouldn´t it be biased hot?
The first impression is a much better sound and way more bass! But the volume is not impressive? And there is no gain/distortion what so ever. Volume almost fades out when toneknob is turned towards the bass side?
The voltages seem fine:
Powertubes:
pins 7: 351 vdc
pins 9: 352 vdc
V1
pins 1,6: 150 vdc
pins 3,8: 1,25 vdc
V2
pins 1,6: 253,258 vdc (should they be exactly the same?)
pins 3,8 : 66 vdc
B+ is 356 vdc
1st 33uf cap 356 vdc
2nd 33uf cap 354 vdc
1st 22 uf cap 335 vdc
2nd 22 uf cap 323 vdc
heater voltage (read between pins 4,5 - 9) is 6,8 vac ( a tad high?)
Cathode resistor is 150 ohms, and the voltage across the resistor is 13,2 vdc
Plate voltage pins 7-3 reads 338 vdc
Using the weber bias calculator gives 14w/41ma per tube
i know this is hot, but shouldn´t it be biased hot?
The Deluxe Reverb had an OT with 6.6K impedance... the Princeton Reverb is the one with the 8K OT.... If it truely is a Deluxe Reverb OT you've likely got an OT with a 6.6K secondary...
just sayin....
TT
just sayin....
TT
I didn't realize that?
But still, it Would be fine i Think, an i'm unsure if you're suggesting the OT is the reason for the low output?
But still, it Would be fine i Think, an i'm unsure if you're suggesting the OT is the reason for the low output?
Bagudan: I need to correct myself here! To reflect back to primary at 4k, if the OT is Nominal 8k: then a 4 Ohm speaker load connected to the 8 ohm secondary is the correct method. The output and OT will be OK as they normally can accept that variance, even the 8ohm in the 8 ohm jack.
The loss in high frquency may be the number of turns is being higher on the new transformer. That will affect directly inductance and the HF rolloff.I don't think the spitfire has any NFB to compensate for that.
If you don't have a 4 ohm speaker than maybe you can get another 8 ohm and parrallel?
Well, it´s allways something stupid isn´t it.
The 25uf electrolytic and the 1k5 resistor coming from pins 3 and 8 from V1 wsn´t properly grounded. I did put the wire in there, but it wasn´t even soldered in?!!? Don´t know how I could have missed that.
i must say it sounds excellent!
I did both the 5e3, and the 18watt, and really like them. But I absolutely love being able to dial in some great preampdriven gainier tones at practice-home-level.
Thanks for the help and replies.
By the way. It sounds great with and 8 ohm speaker on the 8 ohm tap. Excited to see what difference it makes, If I put a 4 ohm load on there instead, as the cirquit is intended for..
The 25uf electrolytic and the 1k5 resistor coming from pins 3 and 8 from V1 wsn´t properly grounded. I did put the wire in there, but it wasn´t even soldered in?!!? Don´t know how I could have missed that.
i must say it sounds excellent!
I did both the 5e3, and the 18watt, and really like them. But I absolutely love being able to dial in some great preampdriven gainier tones at practice-home-level.
Thanks for the help and replies.
By the way. It sounds great with and 8 ohm speaker on the 8 ohm tap. Excited to see what difference it makes, If I put a 4 ohm load on there instead, as the cirquit is intended for..
Very cool, I am happy you were able to run it. Anyways the Mercury Mag was the way to go..that is a high end OT. And yes it usually is something silly, at least from my experience!
Tried cranking All knobs on the spitfire today.... Sound awesome, and there is plenty gain!
But i noticed the el84s glowing brighter along with the applied signal? Meaning, when I strum the guitar hard, they glow a bit brighter? Have never seen something like that before?
Is this bad or ok?
They are not redplating when i turn of lights, and bias seems ok?
But i noticed the el84s glowing brighter along with the applied signal? Meaning, when I strum the guitar hard, they glow a bit brighter? Have never seen something like that before?
Is this bad or ok?
They are not redplating when i turn of lights, and bias seems ok?
Hi Shane,
I've seen that a couple of times - heaters getting brighter with signal peaks, without visible red plating. Best explanation I could think of was that the anode was getting warmer, so the heater in the middle did too.
Maybe try looking through an infra-red camera, some cameras do that, or a cheap ccd camera with the IR filter (bit of plastic just behind the lens) pulled out?
That maybe from the slight hot bias? If the OT secondary is reflecting back more impedance to primary , using an 8 ohm speaker on it IMO I don't think it will be problematic. But there may be some fluctuation with respect to bias being driven relatively higher momentarily with large signal (playing hard) or might be dissipating right on the edge of the max rating of the tubes. Just suggestions, any comment welcome. Without going back to references, is the bias of those tubes normally set for 60% to 70% of max dissipation? if you have bit higher OT primary impedance it should be lowering the plate current ..am I right? Maybe there is something else going on. Would check back on the grids to see if dc is stable while the amp is driven.
I'm away from home the next ten days or so. Will check the DC on the grids, when playing, when I get home. Thanks for the replies....
They are biased quite hot by the way... Dont remember exactly, but maybe 14 watts Per tube , 41 ma Per tube.... Something like that... Will check bias also when I return.
They are biased quite hot by the way... Dont remember exactly, but maybe 14 watts Per tube , 41 ma Per tube.... Something like that... Will check bias also when I return.
Well, back form holiday
It´s not the heaters glowing brighter! It´s something else? It´s not the glowing that bothers me, but more that I can hear thers sometbing going on thats not right?
When I strum a chord, and let it ring out, the tone kinda buzzes when it fades out.... And the amp is not stable....sometimes the volume drops.
I checked the dc on the pins when playing hard:
Plate voltage is unaffected
Pin 3: The voltage is around 12vdc, but I read up to 16 volts (only momentarily) when playing loud and hard!
Pin 9: Here there is a drop in voltage (up to 20 vdc) when playing
Pin 2: When not playing, I read aprox 12 mv, when playing hard, the meter freaks out a bit, and sometimes show 40 mv
The bias by the way: 13 watts per tube (38ma)
I don´t think I have wiring problems, but theres one thing I´m unsure of? Is it a problem that the cathode resistor I´ve used is wirewound?
It´s not the heaters glowing brighter! It´s something else? It´s not the glowing that bothers me, but more that I can hear thers sometbing going on thats not right?
When I strum a chord, and let it ring out, the tone kinda buzzes when it fades out.... And the amp is not stable....sometimes the volume drops.
I checked the dc on the pins when playing hard:
Plate voltage is unaffected
Pin 3: The voltage is around 12vdc, but I read up to 16 volts (only momentarily) when playing loud and hard!
Pin 9: Here there is a drop in voltage (up to 20 vdc) when playing
Pin 2: When not playing, I read aprox 12 mv, when playing hard, the meter freaks out a bit, and sometimes show 40 mv
The bias by the way: 13 watts per tube (38ma)
I don´t think I have wiring problems, but theres one thing I´m unsure of? Is it a problem that the cathode resistor I´ve used is wirewound?
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