What Fender amp should I build?

[ElPedro]

OK, this one is going to open up a can of worms, I suppose.  I'm looking for another Fender model to build.  I've done the champ, the 5e3, the twin and the princeton reverb. Here are the criteria:

- Fairly light
- Up to ~40 W output max
- Late breakup
- Takes Pedals well
- Reverb circuit a plus
- Good tone - not too bassy like the princeton, not shrill, either.

Maybe a Fender doesn't fit the bill here and something else does? I am open to suggestions. 

I've also been toying around with the idea of building a channel switching amp.  Any suggestions there? 

[P Batty]

I've always wanted to build a single channel 6G12a Concert but haven't got the nerve. I've got a friend who bought one new in '64 and he's never felt the need to buy another amp—he's still gigging with it! No verb, although you could build it into the first channel.

[mresistor]

Well, I just built a single channel Deluxe Reverb on a Hoffman turret board and I really like it. I actually like it better than my Hoffman Princeton Reverb. It really has that sweet creamy Fender sound in spades.

[Toxophilite]

Vibrolux or vibroverb would seem to meet your requirements

[p2pAmps]

Good ole Bassman Head is a great one to have in the arsenal...

[DummyLoad]

super reverb head.


--pete

[tubenit]

I'll offer a different idea.  Some of the guys on the Amp Garage are building a Small Special which is a Dumblish amp with clean and no overdrive. It has some similarities to my Tweed BluezMeister clean channel which is the one of the best sounding clean tones I've ever had.  And when the PAB and midboost is engaged,  it has almost the equivalent overdrive as the overdrive channel (without PAB or midboost).

I've simply added a one tube reverb idea to the mix.  I think this would make a very nice clean tone amp that takes pedals well and still allow some reverb or use of delay in a passive FX loop.

So, I think you'd get a warmer Fenderish tone but with added PAB, mid-boost and passive FX

With respect, Tubenit

[ElPedro]

Good ideas so far!

Tubenit - do you have a turret board layout for what you proposed by chance?

Has anyone done relay channel switching amps? I'm considering doing a stout with the relay channel switch added gain mod on the el34 info board as well.

I'm new to reading schematics.  And transferring a schematic to a turret board layout isn't something I think I could pull off at this point.  I'm learning, though. Each amp that a build from a layout is finalized by a study of the actual schematic - I'm starting to figure out where things would go and being able to visualize things better.  Also reading a tube amp design book, but ...yawn, it's tough for me to dig into it. I usually end up falling asleep.  No offense to you guys that live for that type of reading - I fall asleep reading anything.

[tubenit]

Tubenit - do you have a turret board layout for what you proposed by chance?

I have one that I can easily modify for which I am willing to do IF you are significantly interested.  IF you are simply curious, but not interested then I'll pass on doing the work for you.  It would take me about 20 min to finish a layout for you and I'd be happy to do so IF you are genuinely interested. I'll try to post it Saturday if you want?

With respect, Tubenit

[tubenit]

Has anyone done relay channel switching amps? I'm considering doing a stout with the relay channel switch added gain mod on the el34 info board as well.

Have you checked ARCHIVES?    Lots of info, schematics, layouts, pictures on relay switching there.

http://el34world.com/Forum/index.php?topic=14348.0

With respect, Tubenit

[ElPedro]

Tubenit -

As I am new to amp building and the el34 board, I am genuinely interested in EVERYTHING.  At this stage of my building experience, I don't have a thing to offer you in return for your efforts other than my sincere thanks and that I will build this amp to the best of my ability.

If you have time to work up a layout this weekend, that would be great. But I understand it's your time being spent on this layout, and it's holiday time - so no need to feel like you have to do this asap!

Thanks also for the link to the DMars discussion. A lot of it is over my head at first glance, but I'll keep going through it until I get a good understanding of it.

[tubenit]

I am happy to help out.  I will post the layout today Sat or tomorrow at the latest.

Tubenit

[tubenit]

ElPedro,

OK,  here is a schematic with a layout for the amp.  I simplified a few things for you on the original schematic.

You can do footswitched relay switching ............ OR simply use a mini-toggle for the PAB (preamp boost) and midboost. I do recommend having the PAB and midboost and think you'd find it useful.

The schematic is fixed biased.  You can change this to cathode biased if you wish using a 270R/10w cement resistor and a 100uf cathode cap. If you build it cathode biased (which I prefer), then you can change between 6V6 or 6L6 without rebiasing.

You can use a 2.2M reverb pot IF you want more reverb.  Using a 1 M reverb pot, I usually have it set around "3".

Hoffman should carry a PT and/or OT that would be suitable.  A 5F6-A Bassman OT or ProReverb OT would work well, IMO.

I'd recommend using Orange Drop PS or Hoffman's yellow Jupiter caps for the preamp and then either regular Orange Drop or Mallory 150's in the LTPI to the power amp.

PLEASE do NOT feel obligated to build this simply because I drew it.  A tradition Fender amp may be a better match for what you want?  I simply mentioned this as an alternative to consider.  With whatever you build, please keep us posted on how the build is going?

I included an editable Express SCH schematic and layout for you.  You would need the FREE download of Express SCH to use it. There are 100's of editable schematics and layouts done in the SCH format.

Check for errors!  Any discrepancy between schematic and layout - go with the schematic!

With respect, Tubenit

[ElPedro]

Thank you thank you thank you !!!!!  I am over at my moms installing some of her new Christmas gifts.... Can't wait to review the drawings and get started!

[ElPedro]

Tubenit -

Thanks again for the layout and schematic!  I've been looking at both, and have a couple of questions so far:

1) In the power rail, on the schematic, there is what looks to be two 220K resistors and two (?) 220uf/350v caps.  What are those for?

2) I'm a little confused on the NFB - I don't see it on the layout, but I see it on the schematic. Should I just squeeze that circuit onto the layout board, maybe off to the left/top of the .1cap and 24k resistor junction?  What is the "2kb" in the NFB circuit on the schematic?

3) I don't want to assume, but comparing the schematic vs the layout - the A's, B's, C's etc transfer directly from one to the other? It appears so, but again, don't want to assume.

[tubenit]

1)  The resistors  drain the filter caps.  The filter caps in series increase voltage rating. IF you prefer to simply use a 40uf/500v cap, then do that.

2)  Any discrepancy between layout and schematic, go with schematic.   Yes, add NFB.  2kb potentiometer.

3)  Compare layout with schematic.  Any discrepancy, go with schematic.  The nodes on the B+ rail on the schematic should correspond with layout

With respect, Tubenit

[PRR]

> two 220K resistors and two (?) 220uf/350v caps.  What are those for?

I don't know, but suspect, that the B+ here can rise over 450V.

Ordinary electrolytic caps only go to 450V. If you go over that, the cap blows-up.

(Film-caps and special production e-caps are available in higher voltage but at higher prices.)

When you need a large cap for over 450V, it is often economical to stack two caps in series.

Two 220uFd 350V acts like 110uFd and maybe 700V.

"Maybe", because there is no assurance the voltage will split exactly 50:50. The voltage split is related to cap DC leakage. Caps have a spec for maximum leakage but are often much better than spec. You could pick up two "identical" caps, one leaking 100uA and the other leaking 10uA. The voltage would then split about 10:1. At 500V, maybe 50V on the more-leaky cap and 450V on the less-leaky cap. That would be bad for 350V caps.

Actually as the voltage rises over 350V, the less-leaky cap leaks more. They tend to equalize. But this is not assured, and the one cap will be hovering too close to its blow-up rating.

Two equal resistors will split voltage equally. 220K across say half of 500V is over 1mA or 1,000uA. Any little 100uA difference in cap leakage is overwhelmed by resistor current.

Note that these 220K resistors will idle at 0.3 or 0.4 Watts. For "forever" life you want to double-size: 1 Watt.

Yes, the entire "Presence" network on the schematic is missing from the layout. Exercise for the Student: figure out how much was omitted, move-over half the layout so you can fit-in the overlooked parts. If you are very unsure, leave 3 rows of blank turrets and then install the parts. I suspect it will be very much less than 3 rows. In a bind, I would air-wire 390r and 2uFd on the back of the pot. But that's less road-worthy.

There is an error on the schematic, PT HV secondary. The 50V tap is shown dotted (connected) to the 350V(?) winding. That will be a quick smoke. There should be a cross-over here.

[ElPedro]

I guess I don't understand why those caps are there.  To me, it looks like b+ is coming down the line, and connects to the standby switch.  But prior to hitting that switch, a branch goes off and runs through two caps/resistors straight to ground. That's how I read the schematic. If that is the correct read, would someone please explain why that is needed, as I thought the filter caps were downstream from the standby. If I have read the schematic incorrectly, would someone please tell my mistake?

Thank you all.

[sluckey]

Many amps put the first node filter cap between the rectifier and the STBY switch. Like these popular Fender amps...

     http://el34world.com/charts/Schematics/files/fender/Fender_DELUXE_REVERB_AB763.pdf

     http://el34world.com/charts/Schematics/files/fender/Fender_Super-Reverb-AB763-schematic.pdf

     http://el34world.com/charts/Schematics/files/fender/Fender_Twin-Reverb-AB763-schematic.pdf

[ElPedro]

Thank you, gentlemen.