New Project: Assembly of a tube preamplifier

[BuzzK]

I'm going to start another tread and post as I build a custom preamp.

Awhile back I bought a few preamplifier boards from John Broskie of TubeCad. They are incredibly well-made, and I like the design philosophy. I've used his Aikido line stages elsewhere, and they rock. So I thought I would use them.

One problem is that the boards are physically large. It's hard to find a enclosure that is large enough to accommodate them.

I had an old (and not especially good) distortion meter that had a large case, and decided to rework it.

Attached are pics of the before, after I stripped it.

 

[BuzzK]

And here are a few shots of the reworked enclosure.

 

[Dave451]

Nice work!

 

[AudioWizard]

I like your recycling efforts! Very clean.

 

[Tinkerbelle]

Very nicely done!

 

[BuzzK]

This project gave the metal fab shop all kinds of headaches. I'm lucky that the co-owner is a good friend.
It's hard to imagine what a new prototype of this quality would cost.

I've got to order parts like the RCA jacks, but I have most of them and will be ready to assemble it.

The next step will be to have a trophy shop engraved fill the lettering. I can mount the torrid and some boards in the meantime.

 

[BillWojo]

What boards are you using? I have an early version of the Aikido octal line stage board that has all the octal tubes lined up in a row. Your quite right on the quality of the boards, nicest I've seen. I have been slowly picking up parts for this build. It uses 6SN7 or 12SN7 tubes.
Will follow along with your build.
Right now I'm using a VTA-SP9 preamp that uses 12AX7 tubes, 4 of them. I was told it's based on the Aikido design and I love it.

BillWojo

 

[rudedogg]

Cool project. Watching with anticipation! I have a couple Aikido preamps built by AKers.. absolutely love them!

 

[BuzzK]

What boards are you using?
BillWojo

I'm using the Aikido Stereo 9 Pin PCB and the Tetra All-In-One Stereo Phono PCB.

I have a number of other Aikido PCBs as well. Must have been on a buying frenzy.

 

[BuzzK]

I think this is how I might place the boards and toroid.

Also I want to have input switching with relays (small PCB)

 

[dcgillespie]

As usual, your cabinet and metal working are positively first rate, from conception to execution. Nice you're using a toroidal transformer on this project. DC on the heaters?

Dave

 

[BuzzK]

As usual, your cabinet and metal working are positively first rate, from conception to execution. Nice you're using a toroidal transformer on this project. DC on the heaters?

Dave

Thanks, Dave. DC on all heaters -- that's pretty standard with Broskie PCBs.

 

[BuzzK]

Initial Design Considerations & Misc.

In order to keep noise down, I decided to use a toroid power transformer. AnTek has a nice selection, and they are pretty cheap – especially when compared to Plitron, which I've used before. I also wanted to have input switching by relays, to reduce the amount of signal wiring traversing the chassis.

Because I'm lazy – and because I already had one on hand – I intend to use a volume control with an IR remote control feature.

I changed my mind on the layout during the time that the sheet metal guys were redoing the chassis. They had installed some PIM stand-offs that are now in the wrong place, so I'll have to drill them out. There is a hole for the toroid mounting bolt on the right of the chassis, but I'm going to keep it on the left. The plan is to keep all of the A/C mains stuff on one side, away from the audio signal paths. The pic below gives a rough idea of the revised layout.

The Tetra phono PCB is a constant-current-draw amplifier (CCDA) that employs no global feedback. You can choose from among a lot of different tubes, such as the venerable 12AX7; or the 12AT7, 6N1P, 6DJ8, etc. in pairs or in various combinations. I decided I'm going to use all 12AX7s (I can hear groans from the 'X7 haters). They sound fine to me; have plenty of gain for the application, and the filament current is low. The PCB features an on-board regulated DC supply for the filaments. They recommend a B+ of 250-300V for the 12AX7s.

The Aikido line stage lets you pick tubes as well. I've built them with 6CG7s before and thought they were fantastic -- this line stage doesn't seem to add any coloration to the signal. There are some other features of the design that are nice -- you really can't go wrong with Broskie's design here. The same B+ works out well with the 6CG7s.

The B+ isn't regulated on the Tetra; I think I'm going to piggy-back the Aikido's HV section with the Tetra rather than use a separate AC/DC supply.

–– More later ––

 

[BuzzK]

Initial Design Considerations, con't

Transformer

The AnTek AS-1T200 is a 115/230V, 100VA transformer that delivers 400 VCT and has two 6.3V filament winding @ 3A each. My mains runs about 124 VAC, which will yield about 8% higher voltage on the secondaries. 200 x 1.08 x 1.414 = 305V after rectification. That should work for both the phono and line stages.

Broskie recommends a 1.8X current factor for the filaments. In other words, "the heater supply power transformer must offer at least 1.8 times more current than the heaters will draw." No doubt this is a good rule of thumb, but I don't think that most of the earlier tube designers designed in this much margin.

If I use separate windings for each section, it shouldn't be a problem with the phono board: 0.3 x 4 = 1.2A draw for the 12AX7s. The 1.8 factor requires 2.16A, plus some current that will be dissipated by the regulator.

6CG7s draw a total of 2.4A -- there is only a margin of 1.25 on the supply (vs 1.8). It's probably not going to be a deal-killer, but there is certainly no room for current that would be drawn by a regulator.

I could parallel the two 6.3V windings, and hope that the current draws through them would be balanced. But instead I am going to use an unregulated DC supply and 8CG7s which are plentiful dirt cheap (I just picked up 13 NOS for under $50, delivered). They draw .45A at 8.4V (same VA). So one transformer winding provides 18.9 VA and the filament total is 15.12. I think we'll be OK in spite of Broskie's proviso. I welcome any suggestions or rebuttals.

Rear Panel

I've got to label the stuff on the back panel, including 4 inputs and the line out. I'll draw out a template and take the panel to my local trophy shop. They can engrave it so it's really permanent. I used to use Datak rub-on labels and protective spray, but those aren't available and more, and the more expensive route looks a lot more professional.

In my prior preamp, I included a step up transformer and switching scheme for both MC and MM phono. But I only have an MC cartridge, and I can always use an outboard step-up transformer if my need changes, so there will be only one phono input this time. I am much more likely to have additional line-level sources.

What should I name the other three inputs? Times have changed . . . and so have the things that we connect to our systems.

I'm thinking: CD, PC & Aux? or CD, Tuner & Aux? or Input 1, Input 2, Input 3?

Thoughts?

 

[BuzzK]

Problems, problems!

John Broskie is an exquisite, talented designer. However, I wish he had spent a bit more time and effort on the documentation. It's clear that he used a lot of cut-and-paste from some of his other manuals; part numbers on the PCB often don't match the print. So you have to check and double-check.

Ironically, his "cut and paste" way of doing stuff is going to save my bacon. Here's how:

It was [initially] unclear from his docs that you couldn't use a 6.3 VAC source with the 12AX7s -- there is no way to parallel the filaments on his board -- pin 9 is left floating.

However, Broskie also offers nice (separate) filament supply kits that feature full wave or voltage doubler rectification.

A comparison of the two reveals the similarities between both the circuits and the PCB layouts.

Apparently, I can configure the Tetra into a voltage-doubler configuration. And if so, that's an undocumented feature!

 

[triode17]

I hate to be the fly in the ointment but you are assembling a preamp. Great work, beautiful box, but i built one from scratch. i do admit to using the Aikido cathode follower but i built that as well from scratch. I purchased all the parts and built them on Vectorboard with "Flea Clips", Belden bus wire and Teflon insulation. To me, that's building something. No offense meant.

 

[BuzzK]

I hate to be the fly in the ointment but you are assembling a preamp.

Thank you. I stand corrected, and fixed the title accordingly. Now please help me with some of the questions I asked - no one has responded yet.

 

[triode17]

It was [initially] unclear from his docs that you couldn't use a 6.3 VAC source with the 12AX7s -- there is no way to parallel the filaments on his board -- pin 9 is left floating.

Buzz- Glad we can be 'friends'. The heaters are connected for a 12v supply. That's why pin 9 is left floating.You may have to voltage double a 6.3 vac output into 17.7Vdc. Then , you can regulate it if you'd like with a 7812, I would.

 

[triode17]

200 x 1.08 x 1.414 = 305V after rectification.

Are you using a tube rectifier? If so, you won't get a 1.41 transfer ratio. More like 1.2.

 

[kward]

Can you jumper a wire in to pin 9 of the preamp board so you can use a 6.3V supply? Don't know...just wondering if that might be another option.