Amp mod(s)

[synthecks]

Hello, first time poster here. I'm looking for some help with making my fender PA135 amp more like a guitar amp.
I would like to get this amp to sound like a old Sunn model T.
I see some similarities when comparing the schematics but dont know what changes (new power tranny, rectifier ect) would bring me closer to what I want this amp to sound like.
Im also just branching into amps from pedals so a lot is still new to me.
Below are links for the schematics of both amps.
thanks in advance,
-j

http://sunn.ampage.org/site/schematics/modelt.gif
http://www.prowessamplifiers.com/schema ... pa135.html

[synthecks]

Anyone know how to change this to tube rect?

[Geek]

Anyone know how to change this to tube rect?

The current draw of 4 x 6550 would make that a bit impractical. But two paralled GZ34's that have their own heater transformer would do the trick.

Keep in mind the PIV here is pretty big... you may wish to put them in series with SS rectifiers.

Cheers!

[synthecks]

I was talking about the fender to tube rect.
And Id need it in laymens terms.

[DeathRex]

Hello, first time poster here. I'm looking for some help with making my fender PA135 amp more like a guitar amp.
I would like to get this amp to sound like a old Sunn model T.
I see some similarities when comparing the schematics but dont know what changes (new power tranny, rectifier ect) would bring me closer to what I want this amp to sound like.

The fender seems to be a good amp. Though they seem very similar, to get the SUNN sound you migh have to do several things: install 6550 tubes, replace power transformer, replace output transformer, rewire for 12AX7s. Basically replace the guts with SUNN guts. But that would destroy close to a $750 amp.
To replace the silicon diodes with tube rectification, would take 2 GZ34s, because 1 can't handle the current. Maybe cheaper to get 2 X 5U4GB, plus you'll need a 5 volt 4-6 amp transformer. 4 X 6550s will use up to 280ma. They should idle at 200ma at 500 volts.

You might be able to clone one. Look around the internet. I've found a post that said the early gen one might have been built with dynaco parts and you can use a A431 output transformer for up to 120 watts. With the 2 X 6550s, the secondarys will change to 8ohms, 4ohms, and 2ohms.
http://sunn.ampage.org/sdp/index.php?ac ... pic=3796.0
http://store.triodestore.com/a4dyoutr43oh.html

[synthecks]

Thanks DeathRex. Yeah, the more Im comparing these 2 the more content I am with what I have. I feel like I'd like the warmer compression Ive heard about with tube rects, but feel its above my skill level.
Probably simpler for me to try to clone one someday.

thanks again.

PS I was reading something about a "death cap" with old fenders and someone suggested this amp had one can some one confirm?
Lastly is there something different about this amps bias circuit from a guitar or bass amp?
There is a output tube matching trim on the back. So I adjust this till I hear minimal hum?
Does this mean I can basically plug in any 6L6 tube and turn the trim to adjust the bias between the 4 6L6s?

[synthecks]

You might be able to clone one. Look around the internet. I've found a post that said the early gen one might have been built with dynaco parts and you can use a A431 output transformer for up to 120 watts. With the 2 X 6550s, the secondarys will change to 8ohms, 4ohms, and 2ohms.
http://sunn.ampage.org/sdp/index.php?ac ... pic=3796.0
http://store.triodestore.com/a4dyoutr43oh.html

How difficult would it be to switch out a OT like this for mine?
Its probably not as simple as desolder and remove old and resolder new in exact or similar wire configuration is it? It would be nice to have a selectable ohm on my amp.

[DeathRex]

I'm not into guitar amps. Just never tried building or fixing one, except for a transistorized PeaVey.

According to your schematic, your Fender doesn't have the "death cap". Appears the death cap is a X1/Y2 safety cap. But back then it might have been just a regular film cap. They are connected across the AC line input, or from AC to chassis. It helps filter EMI I do believe.

The "Output Tubes Matching" pot adjusts the bias between the two sets of 6L6s. I don't know how you check the bias on this, but you would want the tubes' bias to be equal, not necessarily the bias voltage, but the current, for all 4 tubes to be the same. If it's off, you will run 2 tubes harder than the other 2 and have to replace them more often. You do not adjust it for hum balance. You do have a hum balance pot, it's at the bottom of the schematic.

[DeathRex]

How difficult would it be to switch out a OT like this for mine?
Its probably not as simple as desolder and remove old and resolder new in exact or similar wire configuration is it? It would be nice to have a selectable ohm on my amp.

You'll have to see if it will mechanically fit. There should be a mechanical diagram for it somewhere. Other than that, you'll have 5 wires on the primary to connect and 4 on the secondary to put somewhere. You'll have to find someone that can solder it in and desolder the old one. If you can solder then it will be cheaper, just be careful of the very hazardous voltages on this one. Even with it off it might take a while to discharge 500+ volts. Then it's $140-150 for the transformer. You can always ask Triode about it too.

[synthecks]

Thanks for confirming a few suspicions(death cap,OT match pot).
Yes I solder, have built a few pedals. Maybe I'll replace that OT someday.
Ive read that if you have good ears you can year less hum from your amp when you get the OT match pot set equal between the sets of tubes.
I checked this yesterday and I can hear it.

[DeathRex]

Thanks for confirming a few suspicions(death cap,OT match pot).
Yes I solder, have built a few pedals. Maybe I'll replace that OT someday.
Ive read that if you have good ears you can year less hum from your amp when you get the OT match pot set equal between the sets of tubes.
I checked this yesterday and I can hear it.

If you have a multimeter you can set the hum balance to the middle, then set the bias to lowest hum. I suppose if the hum balance is off, then the output tubes matching can be off. You can see the same circuit in a Eico HF-89, except there are test points on the cathodes of the output tubes. You want the voltages to be the same for each tube. Can't do that for yours, because you don't have the test points. I found the manual for the Fender, but it doesn't talk about adjustments. Maybe a local repair shop can tell you officially how to adjust it.

[EWBrown]

Another approach for tube rectification modification, would be to use some TV "damper diodes", these are half-wave rectifier tubes, so two will be needed. These come in 6.3VAC, and higher voltage, lower curent filament ratings, too, and they are generally dirt cheap. These damper diodes can handle some impressive currents and they generally have a lower forward voltage drop than do most fullwave (two plate) rectifier tubes.

To add this to your amp, would require two tube sockets, and an extra filament transformer - the existing power transformer's 6.3VAC winding won't handle the extra 3 or 4 amps of filament current.

A second, amd simpler, approach would be to use a single damper diode after the cathodes of the SS rectification. This would do two things for you: Offer a slower "traditional tube rectifier" start up and allow for some B+ voltage sag, which is an important element in getting that "right sounding" tone.

/ed B

[synthecks]

Thats too easy to sound good, haha.

A second, amd simpler, approach would be to use a single damper diode after the cathodes of the SS rectification. This would do two things for you: Offer a slower "traditional tube rectifier" start up and allow for some B+ voltage sag, which is an important element in getting that "right sounding" tone. /ed B