RX/ES Help

[jon]

A little while ago I built a RX/ES with some parts I had laying around. Obviously the chassis did not lend its self to a traditional layout so I just built it ptp. Since I built it I've been trying to figure out why mine does not sound like the clips I have heard. Mine is amazing to the halfway mark on the volume after that it begins to take on a harshness to the overdrive that is not entirely pleasant to my ears. The first thing I tried was lowering the plate voltage a little. I used an Express/Liverpool transformer, originally I was running on the 300V taps but I have now switched to the 260V taps. I have also adjusted the cathode resistors to compensate (120's instead of 150's) I am currently running 94% PD on the power tubes. I also lowered the volume pot from a 1M to a 500k thinking that might help. Here is the layout that I based my amp off of. Any suggestions would be great.

My two thoughts have been to:

Throw some signal away before the pentode gain make up, ie: split plate load on the triode. And I have also though of changing the negative feedback value.

[IMG:800:600]http://i82.photobucket.com/albums/j250/ ... G_0200.jpg[/img]

[rsalinger]

Is yours a clone of the "old Prescription" or the new re-designed, current production model?

[jon]

Mine is the RX/ES with the EF86 in the gain make up stage. See the attached layout.

[tictac]

Unfortunately trying to duplicate a particular amp of any kind is more than just following a schematic. Proper layout and lead dress is critical, if you want to make an amp that sounds like a Fender then the layout should be like a Fender too. parts selection can drastically affect tone as well.

Sound like you have some bad oscillation going on and that can be one of the most frustrating things to troubleshoot and fix. If you have access to a gut shot of the Rx/Es look to see if there's any shielded cable on any of the preamp tube grids, that's a start... Even the position of a capacitor that's in the signal path can cause the problems you describe.

Where did you get you're schematic? Is it correct?

If you're not familiar with proper layout and lead dress techniques this would be a good time to study up; search the forum and read up; look at the nicely drawn layouts you can find in the file section and try to understand why they are constructed that way.

I've built an amp or two with "self-designed" layouts that I thought were really good only to find out after plenty of tube swapping and parts changing that the layout was actually the problem...

hope this helps...

TT

[jon]

Interestingly I have found that generally "Board based" layout seem to be more critical to part position and lead dress. The PTP amps I hve built in the past all seem to be quite and oscillation free.

Recently I built two (2) Creme Brulee amps one PTP using junk bin components (Like this amp) and one neatly lay'd out on a board with really clean lead dress. They both sounded identical. The layout I posted is in agreement with the images available.

[Gaz]

It' somewhat true about lead dress, but it won't make or break amp unless there's something really wrong like an oscillation. Have you seen gut shots of Dr. Z's, Tictac? They are all different, somewhat free-form board layout, and dare I say, sloppy. I think your problem lies elsewhere.

[jon]

Unfortunately trying to duplicate a particular amp of any kind is more than just following a schematic. Proper layout and lead dress is critical, if you want to make an amp that sounds like a Fender then the layout should be like a Fender too. parts selection can drastically affect tone as well.

Sound like you have some bad oscillation going on and that can be one of the most frustrating things to troubleshoot and fix. If you have access to a gut shot of the Rx/Es look to see if there's any shielded cable on any of the preamp tube grids, that's a start... Even the position of a capacitor that's in the signal path can cause the problems you describe.

Where did you get you're schematic? Is it correct?

If you're not familiar with proper layout and lead dress techniques this would be a good time to study up; search the forum and read up; look at the nicely drawn layouts you can find in the file section and try to understand why they are constructed that way.

I've built an amp or two with "self-designed" layouts that I thought were really good only to find out after plenty of tube swapping and parts changing that the layout was actually the problem...

hope this helps...

TT

As it turns out the EF86 gain make up is the issue. I am getting massive oscillations in that section.

While I hate to do this, I think I am going to backup 10 and punt. I am planning to rebuild using mike's layout.

[tictac]

I have a few gut shots of the Rx and all the ones I've seen have at least two shielded grid leads...

Obviously lead dress is not the only issue but sometimes shielding the grid leads or even moving a lead a few inches can make a big difference...

TT