Help me mod my tube amp

Start by swapping out the signal tubes?

Do you run the amp in triode mode? 

Will the amp accept a 12BH7a where the 12AU7 is? 

There's a discussion about some mods here that might be of interest. 

Two main areas you can improve. 

A. the tubes. 

B. the cheap film caps used inside the amp. 

pretty simple if you know how to solder. 

1. Upgrade the input tubes to get rid of the Chinese tubes. Plenty of NOS or UOS US and Euro tubes available for these sockets including the industrial versions like the 5814A and 6189s.Same on the 12AX7s. 

2. Replace the 0.1uf metalized film caps on the voltage rails with copper foil caps or similar. 

3. replace the cheap metalized coupling caps with Jupiter copper foil caps or similar. 

4. replace the input tube cathode and plate flameproof resistors with same value MILLS wire wound resistors.  

I think I would start by using Gold Lyon KT77s. Do you really need the power of KT120s?

I modded a Prima Luna Dialogue with Takman carbon film resistors, this what newer production PL uses. Duelund, Jupiter, many other choices for coupling caps, I went with Miflex T:PCU. I preferred EL34 vs KT, these 1990 era Svetlana and SED. NOS on signal, driver tubes, nice power cord. Do all this and you have very nice PL, I still listen to PL I modded with guy I sold it to, this with Spatial open baffle speakers, very nice setup.

A mod is a change to the circuit itself, an upgrade is a substitution of better sounding parts of like value, and a restoration is a return to original condition using original parts.

I start with the power supply, high speed rectifiers, film bypass of electrolytic caps, better cord and internal wires, from there move into coupling caps, screen stability resistors and PIV diodes on any tube rectifiers. Enjoy.

Thanks everyone for the input so far. Several people suggested new tubes. As is probably obvious, I am pretty naive about tubes and open to your suggestions.

knotscottt: Yes, I run the amp in triode exclusively. I do not know whether the amp can use 12BH7a's rather than 12AU7's. I will try to find out.

russ69: I started using KT 120's all the way back when I bought the amp because I was coming from solid state and I was accustomed to bigger dynamics than the stock tubes provided. But I gather from your comment that KT 120's may be overkill?

johnss, sns, and viridian: Thank you for your specific recommendations. I will investigate options for upgrading each of the things you mentioned.

Bryon

You have some very good suggestions so far. Definitely agree that film caps, resistors and power supply mods yield very nice results. I would add to that changing your connections for KLE RCA plugs and binding posts and change the signal wire to high quality, something like VH Audio in airlock dielectric. Surprisingly lots of signal quality lost in those parts and are easy upgrades.

As for other tube options, of the EL34 compatible ones PSVane EL34PH are the best I've found for good detail and 3 dimensionality while still keeping a good snap to dynamics.

Hi , it looks like the conversation has split into 2 directions. One about modding your amp, and the other regarding tube rolling. I don’t have your amp, but here’s my thoughts. Better signal tubes will improve the SQ, but they’re expensive and you can go a couple of directions, clarity Vs softness. This is a generalization and usually comes down to Mullard Vs Telefunken. Again this is a generalized comment and it comes down to what you like and you have to experiment with tubes that are expensive. Now to power tubes, it’s midrange lushness Vs bass. So EL 34 Vs KT 120, and everything in between. Mentioned above are SED winged C tubes. They are hard to find and they are some of my favorites. The Winged C 6550’s are the BEST overall tube for the EL 34 midrange and KT 120 bass I’ve ever used. But that’s just my opinion with my gear in my living room and YMMV. Considering what you have, the above mentioned GL KT 77’s are a great tube if you don’t mind giving up a little low end. That being said the two 12AX’s are the first I’d try. Then the 12AU7’s after that. Then experiment with the power tubes. Now to your actual question, again I don’t have a Prima Luna. However I do have tube amps with SS rectification like you. I just had my mono blocks upgraded by the manufacturer. They replaced the resistors with Takman and upgraded the capacitors to higher quality. In addition to that they added MORE capacitance by strapping extra power capacitors onto the boards. Then with SS rectification they changed the diodes to Hexfreds. So I would find someone that has modded that exact amp and get direction on the values and brands of the items needed and go for it. However if you’re not experienced at desoldering and  soldering, without cooking parts on a tight circuit board then you could have it done. FWIT I grew up with Dynaco stuff and built a few items. I also ruined a couple items based on lack of skill. I hope my ramble provided some guidance considering most people on this site know more than I do. Happy Listening, Mike B. 

Thanks, nlitworld, for those additional suggestions. I will check out the binding posts and wire you mentioned.

Mike B-- Thanks for your observation about the different directions this thread has taken. Both kinds of input are valuable to me. I've used Mullards (can't remember which ones) for signal tubes in the past with this amp and liked them. I think when it came time to replace them, I cheaped out. But I've been taking more of an interest in this secondary system lately, as I spend a lot of time in that room. So I will rethink my former frugality. To my knowledge, I have never heard Winged C 6550's. Do you have the same positive opinion about other 6550's?

Thanks also, Mike B, for the suggestions regarding resistors and capacitors. I have been internet researching potential replacements. Although I have modified preamps and speakers myself in the past, I am going to hire someone to do these modifications.

Mullard Vs Telefunken is kinda like Chocolate Vs Vanilla. It’s personal preference within a particular system. I tend to put 6550’s and KT - 88’s in the same group. I compared the Winged C’s to NOS Tung Sol 3 hole 6550’s and the Svetlana’s were considerably better. I’ve used the Winged C’s in EL 34, 6L6 and 6550, they ALL absolutely rock. I really hate to see them go away. I have KT 66, KT 77, 6550, KT 88, KT 90, KT 120, and KT 150. NOS tubes are GEC KT 66, KT 88, and Tungsol 6550. The GEC’s are a little light In bottom end, but the mids and uppers are fabulous. The best way I can describe the Winged C 6550’s is that they have bottom like Tung Sol KT 120’s and mid/high better than new GL KT 88, but not quite GEC KT 88. Out of ALL my power tubes the Winged C’s are my favorite. They are pretty much unobtainable and the ones at the end were factory seconds. In the beginning they were consistent, graded and reliable. At todays prices Primo 6650’s are worth $200 each. This week I’m breaking in an octet of Psvane KT 88’s, but will probably need to buy an octet of better quality Tung Sol KT 120’s for my project. Unfortunately the cost delivered from Upscale is $1100. I’ve seen a few Winged C EL 34’s for sale and maybe 6L6, but the 6550’s are flipping rare. I’m down to a shoe box of loose, test 80% 6550’s. Disclaimer, I’ve never tried Sovtek power tubes. Happy Hunting , Mike B.

I would add output transformers to try.. Luxman/Tango/other good-ones.. 

I would be careful on trying to find genuine-matched NOS tubes. number of NOS offered these days exceeds former production!