November 23rd, 2011, 08:30 PM
I´m about to start my first build - 5e7 circuit - and i still need to buy some components so i would like some advice.
In place of the presence control i´will put a VVR (to use in apartment situations, when needed)
What is best ?
- Hardwire the presence in position 1 (how do i do this?)
- Leave the amp with no NFB loop (how do i do this?)
I dont know if that matters, the amp will be cathode biased.
November 23rd, 2011, 09:23 PM
Dual concentric pot?
Cathode bias and no NFB, not much of a 5E7 left.
To hard wire the presence, start with a pot on flying leads and play with it till you like the sound enough to be able to live full time with, then measure the resistance from the end that is hooked up to the wiper. Replace the pot with that value resistor, but under the chassis, not on flying leads.
To do away with the NFB, just do not install the wire from the OT to the 12AX7 cathode, the one that connects to the 56k resistor.
November 26th, 2011, 01:42 PM
I would build the amp in stock form with respect to the NFB lop and the PResence....and I would keep the presence on the front panel. The VVR control I would put on the back panel.
Celeste wrote: "Cathode bias and no NFB, not much of a 5E7 left." +1..with feeling. The 5E7 is a great amp as is. IT compresses and sings more than does the 5F6A Bassman but has more 'punch' than does the 5E3 Deluxe. As always, YMMV.....
November 26th, 2011, 02:36 PM
If you've seen/heard clips and this is why you decided on the 5E7 - then build a 5E7.
If you liked the simple cathodyne PI only taking half a triode and the other simple quirks of the 5E7 on paper, then decide on what you want tonally first, then draw up a circuit.
I built essentially the same amp - the 5F4, 5E5-A, and 5E7 are pretty much the same (except for speaker config and preamp tubes used).
I dropped one channel, put in a Bright switch for changing between "Normal" and "Bright" channels. Then installed a 4PDT panel switch to cascade the unused channel's triode. Last night I changed the PPIMV for a Master Volume more suitable for cathodyne PIs.
I also installed a cathode/fixed bias switch as well as a SS/tube rectifier switch.
So here's my $0.02 based on my recent exploits with the same circuit with some of the mods you proposed:
1. Keep the stock Presence control. NFB and Presence are underrated - everyone wants without for a "raw, bluesy" sound, but I dig NFB/Presence. They allow for a wider response - much more bottom, boom, and highs. It's subtle, but worth it, in my opinion.
2. Leave it fixed bias. When I switch to cathode biasing, it's not too terribly quiet... maybe a few DBs less, but essentially the same volume, maybe the big OT is partially to blame.
3. The VVR is unnecessarily complex for this amp. I've built VVRs into a variety of amps, but usually only because other, traditional Master Volumes wouldn't quite fit in a circuit right. VVRs when dialing down a loud 6L6 amp will compromise the sound just like an attenuator, pedal, or MV would. Besides, cathodyne PIs don't add gain like LTPs do, so those 6L6's might not get much of a kick for additional overdrive.
I would suggest using the cathodyne MV recently discussed in some threads last week. printer2 posted a series of schematics - his last drawing is optimal. The only extra parts you'd need are a 1M or 500k pot, an additional 0.022uF coupling cap, and a 10k resistor.
The gain on the 5E7 isn't hard rockin, it's more old school blues and '50s rock, but you've got enough preamp drive for a MV. Like I said I put a cascade switch on my preamp. In bone stock mode (tube rect, fixed bias, maxed MV) the 5E7/5F4/5E5-A sound is great... but when I dial down the MV and switch in a cascaded gainstage, it's in Marshall territory distortion-wise, only with a 6L6 tweed vibe.
Anyway, I figured I'd tell you my results in case it helps.
I use the 5E5-A Pro preamp setup - two 12AY7s into a 12AX7 PI, rather than the more gainy 5E7 Bandmaster or 5F4 Super (one 12AY7, two 12AX7s)
November 26th, 2011, 03:36 PM
FYI - There's some discussion of the best mosfet to use for VVR. I recently used Mouser #511-STW12NK90Z and it stays pretty cool mounted to the chassis with a #532-4180 thermal insulator.
November 30th, 2011, 05:04 PM
Thanks for all the info! After your suggestions i´m convinced about keeping the presence control and dont mess with the NFB.
The mojo kit arrived yesterday (my aunt brought it from Canada to Brazil), Its my first build so i´m picking all the information i can in the internet.
I´m still making it cathode bypassed, mainly because i dont want to re-bias the amp each time i swap tubes and cause it seems to be easier to install the VVR. By the way i´m also now also checking about Cathodyne MV.
One quick (stupid) question: To make it cathode bypassed should i follow the same 5e7 layout regarding the PT wiring, and just leave the blue/red (50v) wire unconnected?