Bazzman

[Lynxtrap]

You should be able to bridge them. The phase is the same at the jacks. If you don't want to, here's the switch for one or the other. I put grid leak resistors on both sides because a shared resistor on the jack side of the switch can pop. You could also just put the grid leak resistor between two throws on one side of the switch so when one side is grounded, the other has one meg referenced to ground. sh

Thanks!
I thought the signals would come out of the preamp out of phase. One goes through two triodes (two inversions), one through a pentode (one inversion).

[martin manning]

I thought the signals would come out of the preamp out of phase. One goes through two triodes (two inversions), one through a pentode (one inversion).

Correct, but then you are applying them to opposite sides of the PI.

[Lynxtrap]

Correct, but then you are applying them to opposite sides of the PI.

Oh yes, I didn't think about that. Does driving the PI from both channels at the same time put any particular demands on the circuit? For instance, do I need mixing resistors before the PI?

[martin manning]

Correct, but then you are applying them to opposite sides of the PI.

Oh yes, I didn't think about that. Does driving the PI from both channels at the same time put any particular demands on the circuit? For instance, do I need mixing resistors before the PI?

No, the PI is your mixer.

[Lynxtrap]

I ordered some parts yesterday. I decided to try and keep the original Fender filter section. I'll split the last line at the preamp to a 330K resistor going to the EF86's plate.

Think I'll start by modding the preamp to Z specifications, to see how it sounds before I do further mods. Though I might make the bias adjustable and put in the KT66 first of all.

Question is, will the voltages go down much by swapping the 100K for a 330K at the plate..?

[Lynxtrap]

Ok, had the amp up and running for a while, until one of the resistors to ground on the heaters blew up, for some reason.

Exploding resistors aside, there is also something funny with the tonestack (I think) on the EF86-channel.
There seems to be a lot of gain lost, very little treble, and the bass pot seems to function mostly as a volume control.
Do any of you experts see any reason for that, if you look at the schematic?

By the way, I decided to go with the stock B+ system, including ss rectifier, to begin with.

[timford]

The 66 is not a trebly amp and that tone stack will be fairly neutral at 12 on the settings, and will act like a boost clockwise from there and more of a normal bass or treble cut on the other side.

It is similar/owes heritage to the james and baxandall type tone stacks (page 240/1 of Merlins preamp book - he also shows a mod putting a pot between the (.022) cap and the treble wiper to shift the frequency response which sounds interesting - he suggests particularly for bass though).

I'll have a look tonight and let you know the value for the cap in the treble in the KT45 (which is a much brighter amp with el34s but nearly identical components). Also the route 66 has an ultralinear OT so will probably respond differently with the same tone stack.

I also still need to see if I can draw up the ef86 load line for the KT45, (and the 66 if I can find out the approximate screen voltage).

[timford]

Oh, and sorry, mine was not an expert response. Just a curious and attempted helpful one.

[Lynxtrap]

Oh, and sorry, mine was not an expert response. Just a curious and attempted helpful one.

I was just pretending to be humble No really, I mostly feel like everybody else are experts compared to me.

Thanks for the response! One problem is of course that I don't trust the schematics I used for the preamp. So I can't say it's exactly like the Z. I don't know of any similar tonestack to compare it to, and I'm not capable of figuring it out myself.
I'll have another look at the James. I remembered that as a bit more complicated.

Oh, I forgot to mention one important thing!
The pots are 250K, not 1Meg, since I used what was already in the amp.

I'll make a new schematic when I have the problems sorted out.

[Roe]

For what I like, I'd add adjustable NFB, decrease screens filtering and screens resistors. To get tighter bass, I'd decrease the .1u caps in the PI to .033u

also, can the PT handle the current draw of the kt66s?

[Lynxtrap]

For what I like, I'd add adjustable NFB, decrease screens filtering and screens resistors. To get tighter bass, I'd decrease the .1u caps in the PI to .033u

also, can the PT handle the current draw of the kt66s?

Actually, I have almost given up on the KT66:s. They would physically sit closer together than I'd think is healthy. Those tubes are big and fat.

Why do you think it would need NFB?

[Roe]

Its a matter of taste. try it with a 100k pot and see what you like

[Lynxtrap]

Do you think 250K pots make a huge difference from 1Meg?

[timford]

I'd keep it as close to the original as possible to start with. Then start doing some maths to see what it is actually doing. I'd try the 1M linear pots for the bass and treble, and also play with the cap between the 2 as well (.022 and .01?). The high pass cap on the kt45 is similar to yours (390pF), but I've seen 250 and 1000 used in similar designs. I'm not sure how much these numbers and their effectiveness is altered by the ef86 voltages etc.

Personally I don't want to step on the good Docs toes, but it is a favorite style of tone stack of mine that I want to fully learn and understand so I can then modify values myself based on different types of preamp pentodes etc and making everything work effectively within my own modified designs.

I'd like to see if I could make a SE z28 type amp too with and for my kids (genesis of idea based on his latest SE maz 8. ), and if I could make it work really nicely, let him know (or make one and send it to him). He'd probably laugh and scoff at receiving his own work back, but it'd be like saying thankyou, and here's a complete working model of one of you own ideas.

Keep us up to date and post some sound clips as you go.

[timford]

And don't give up on the KT66s yet.