20 watt Fixed Bias Tweed Amp

[WRC34]

I just wanted to post about an amp I made recently for a friend.

He ordered a Tweed Twin style cab and two 15 watt Weber Blue Dog speakers, and wanted an amp built into the cab. The cab maker did a beautiful job but the chassis cutout was a bit larger than stock so I had to source a wider chassis. I ordered a 22" long chassis from Seaside and I can't say enough about how great it is!

Originally our talks centered around building him an 18 watt lite iib style circuit. Then, he said he wanted a lot of clean headroom, so immediately my mind said no, you do NOT want a lite iib! He wanted the amp to be around the same wattage/output, so we decided on fixed bias using 6V6s.

I used a Classictone 40-18028 PT and wired up a DPDT switch for use of both HV windings (330-0-330 and 300-0-300). I made him an instruction manual so he knows to power the entire amp down before throwing the HV switch and that he'll need to re-bias afterwards. I read a ton of conflicting information out there regarding PT ratings and whether or not you can run 6L6s in an amp using this PT with the higher HV secondary, and my conclusion after doing some testing is that if you use 6L6WGBs or true 5881s with the 330-0-330 HV winding a reasonable bias setting with a plate voltage that isn't obnoxious (around 425VDC and 32mA) is possible and the result is a very strong, clean tone that sounds huge. I'm sure at max volume with this setup the power supply is sagging a bit and the PT may get warm, but in my experience Classictone/Mag Components/MCI/whatever are similar to Weber in that they tend to over build and under spec their products because they understand they're selling to people who like to blow shit up (guitarists). If anyone thinks this is incorrect please say so, since I am admittedly making an assumption.

I used a choke for the screens along with a vintage 25watt OT made by Northlake. The OT was sold to me as having a 6.6K primary and 4/8/16 secondaries but when I measured it using a variac and DMM I got approximately 32:1 winding ratio for the 8 ohm tap which would translate roughly to an 8K reflected primary impedance. Perfect for 6V6s.

The amp is very, very quiet at idle. It uses octal preamp tubes. The heaters are set up traditional style (AC) and there is no discernable, out of the ordinary noise. When he picked up the amp it had a set of 6V6s installed and a 5V4 rectifier using the lower voltage HV winding. giving about 388VDC plate voltage and 22MA bias current. It sounded loud and clean. He was very pleased, and so was I! Here are some photos:

[10thTx]

Beautiful amp! THANKS for sharing the pictures.

With respect, 10thtx

[DenDanger]

Very handsome. I like the point to point wiring.
And the knobs color matches the speaker frames. How cool is that?
Not to mention the NOS tubes.
I would be prouder than proud to have built such a machine!

Denny