1U Rack Mount Project

Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

Post Reply
Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
meble-kuchenne.warszawa.pl
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

1U Rack Mount Project

Post by Mattallica »

I'm wanting to make a few preamps to cover bases the MTS system cannot. Also a 6V6 power amp because why not?
I feel like this is the next step in my amp learnings as well. I have only tweaked existing MTS modules preamp stuffs so far.

1U is a big constraint. Very few off the shelf transformers fit.
The following is the only drop in transformer I can find, so the design is based around it.
I'd rather not have to design a PCB for those wee low profile transformers (like the kits available from London Power). Save some time and money.

https://www.antekinc.com/as-05t240-50va ... ansformer/

Here is my first cut schematic. It's based on a couple of EF86 (small pentode) circuits.
First is Matchless style with a single pentode, second is Blackstar style with a triode in front of the pentode.
I've substituted the Blackstar pentode set up with the ValveWizard one.
I've added some extra EQ controls and a cathode follower for each circuit.
I've used the leftover triode for gain recovery in an adjustable effects loop (which can be bypassed or paralleled).
Channel switching is done via relays, which i've cobbled together from various sources.
B+ power supply I took from a ValveWizard example and tweaked in Duncan Amps PSU Designer II.

I'm still working on the layout, but in the mean time, I would really appreciate feedback on suitability of selected parts and design from the amp minded people on here.

Also, what do you guys think of using these for the power amp?

https://www.antekinc.com/as-05t320-50va ... ansformer/

https://www.antekinc.com/mp-15w50-15w-o ... ansformer/

(Schematic)

Image

User avatar
jeremyb
Chorus of Organs
Posts: 40878
Joined: Sat Dec 06, 2008 9:03 am
Has liked: 7685 times
Been liked: 4157 times

Re: 1U Rack Mount Project

Post by jeremyb »

Cool I’ve seen projects before using cheap toroidal transformers, seems like a smart move!
Slowy wrote: That's the problem; everything rewarding is just such hard work. Regret takes much less effort.

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: 1U Rack Mount Project

Post by Optical »

Go for it, should be a fun build.
I also have one of the AS-05T320 - 50VA 320V transformers that i'm not using if you want it, flick me a message.

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

jeremyb wrote: Sun Jul 25, 2021 8:37 pm Cool I’ve seen projects before using cheap toroidal transformers, seems like a smart move!
They also look cool AF... but yeah, cost!

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

Optical wrote: Sun Jul 25, 2021 8:46 pm Go for it, should be a fun build.
I also have one of the AS-05T320 - 50VA 320V transformers that i'm not using if you want it, flick me a message.
So the schematic passes an initial QC inspection?
Yeah keen on the transformer! PM incoming.
What did you have it in previously?

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: 1U Rack Mount Project

Post by Optical »

The transformer is new, i was going to use it to build an x88r then changed plans
Schem looks ok. Might want to think about adding an irf820 or lnd150 for the loop/send stage to lower the output impedance, it's super high now but might work ok depending on what's next.

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

Optical wrote: Mon Jul 26, 2021 9:15 am The transformer is new, i was going to use it to build an x88r then changed plans
Schem looks ok. Might want to think about adding an irf820 or lnd150 for the loop/send stage to lower the output impedance, it's super high now but might work ok depending on what's next.
Sweet, thank you for the hint. Reading up on mosfets as an alternative to tubes for cathode followers now.
Would it be wise to have one at the output as well?
I could squeeze in another 12a_7 to do the send and output impedance lowering. I like the idea of using solid state parts if their function and tone effect is negligible though.

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: 1U Rack Mount Project

Post by Optical »

Sorry I wasnt looking properly at the values and should revise my comment - the existing cathode followers arent driving much so the output impedance might not be too bad after all. But yeah for a few bucks of parts they are a good addition if a tube stage isnt available. They are nice and simple to use for driving outputs, just dont use them in positions where they clip (volume control before the gate, not after).

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

Updated EF86 schematic.
MOSFETs added to FX loop and to output.
Brite switches removed, Rob Robinettes sizzle control used for tone controls.
Mid switches added to allow mid bypass and toggle between two cap sizes for bridged T circuit.
FX loop is now footswitchable via relays. Footswitch jack options for 2xTRS or 5 pin din (so a midi cable could be used).
Condensed the relays.

Image

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: 1U Rack Mount Project

Post by Optical »

Note that FET config will be ok for an irf820 but an lnd150 wont work well as a source folllower - best configured as a common source amp with local nfb to the gate for unity gain. Also the 47nf cap AC coupling the fx output might need to be increased a lot (think 2-10uF) depending on whether you intend to connect low input impedance devices in the loop (rack fx or other line level gear)

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

Optical wrote: Mon Aug 02, 2021 9:24 am Note that FET config will be ok for an irf820 but an lnd150 wont work well as a source folllower - best configured as a common source amp with local nfb to the gate for unity gain. Also the 47nf cap AC coupling the fx output might need to be increased a lot (think 2-10uF) depending on whether you intend to connect low input impedance devices in the loop (rack fx or other line level gear)
Thanks Matt, I'll read up on the differences between the two FETs and update schem to state irf820.
Could you elaborate on the cap size? It effects impedance?

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: 1U Rack Mount Project

Post by Optical »

The cap and input impedance of whatever is connected, and that 100k source resistance will form the output corner frequency. E.g. a typical 10k line input impedance and the 47n cap will form a high pass filter at about 340Hz - big bass cut. Changing the 47n to 2uF would make that 8Hz instead.

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: 1U Rack Mount Project

Post by Mattallica »

Updated EF86 schematic.
Revised the mid controls.
Added boost pot to red channel pentode.
Revised loop insert coupling cap values and mosfet labels as per Opticals' advice.

Image

Post Reply