5E8A build

[Cliff Schecht]

Reposting this from my TDPRI post:

I finally got my 5E8-A build up and running after a good month of working on the amp. Usually builds don't take me longer than a week but I just started my research this semester so time has not been on my side.

The chassis was something I ended up recycling, it originally had just a big fan mounted on it. It came from a pile of the same chassis, all with an Eimac ceramic tube and a fan mounted on the chassis (this was one of the few that had only a big fan cutout). I used a metal grate from an old power supply to cover the big fan hole and mounted the massive 8H choke and part of the output transformer to it. I spent a bit more time than usual planning out and drilling this chassis to make sure that clearances were all good and the holes were drilled straight. The eyelet board I made from some clipboard-like material I had laying around and ended up being a perfect fit for this chassis and build overall. The chassis is wider and taller than your standard Fender chassis but is shorter in length, about the perfect size for the 1x12 combo I want to make out of this amp really. Chassis dimensions are 6" wide x 3" deep x 17" long. Typical Fender amps looked cramped to me, this build doesn't .

The power transformer was a massive (11 lbs) Triad I bought on eBay for $20 shipped (won it for $5) which I ordered before I knew I was building this amp. I ended up at the 5E8A circuit for a few reasons, one of the main ones being I had an appropriate chassis, output transformer and to a lesser extent the power transformer. I say the power transformer was less appropriate because while it had a MASSIVE 6V and 5V heater taps, the output voltage was about 450V RMS with a 117V primary. After a full-wave 5U4 rectifier this voltage only ends up at about 300V, much too low for a 6L6 Fender amp. So I ended up doing a hybrid tube/SS rectifier (two 5U4's in parallel for the forward diodes, two 1N4007's to ground for the other two diodes in the bridge rectifier). This ended up putting my B+ voltage closer to 550V, well over the ratings of any of my electrolytics. I planned for this though and used a TO-3 mosfet/Zener circuit (the one at the bottom of the Geofex Mosfet Follies article) to drop the B+ down to about 425V. Instead of inserting this circuit where the PT HV centertap goes, I elevated the ground side of the bridge rectifier. I'll post a circuit below because it's hard to describe with words alone..

My initial attempts at this circuit with a BJT was only dropping my B+ to around 530V and after a good week of playing with different circuits, I ended up at RG's circuit with and IRF330 I found and 10x 24V zeners in series to get the B+ voltage down to the right level. As a warning to to others using a zener amplifying type circuit as I did, the zeners don't really want to break down right at their rated voltage. I ended up with 240V worth of zeners to drop maybe 130-140V, and all of the diodes were set at the appropriate current specified in the datasheet for their zener knee to be effective. Either way, lesson learned. The TO-3 gets hot to the touch after 30 minutes of playing but I think it will hold up fine.

Also my bias circuit ended up being lifted almost verbatim from the Marshall Artist 4203 (I think that's the model) which uses a bridge rectifier as I did along with a fixed bias scheme. If you're using a bridge rectifier with fixed bias then essentially you have to ac couple the HV winding to the bias circuit, otherwise the bias circuit just won't work (too much DC from the rectifier causing the fixed bias supply to malfunction).

I've been playing on this amp the past few days now and I must say I am in love with the sound! It sounds equally great with Sylvania 6L6WGB's, 807's and GE 6L6GC's in the output section (each has a different yet equally great character), 12AY7's in the first two slots (Tung-Sol and GE respectively), a GE 5751 5-star in the intermediary gain stage and a Sylvania long-plate 12AX7 in the PI. Currently I'm playing through a 1x12 loaded with an EV SRO (8 Ohms) and find myself switching between the normal and bright channels with my Tele to get a nice variety of clean and distorted tones. The cleans are tight and spanky with a great balance of tight lows, sweet mids and smooth high end. The distorted sound is again punch with the same mix of lows mediums and highs, full sounding without being harsh in any way. The leads tones available would be equally usable for rock and blues. I prefer the bright channel for cleans and normal channel for dirty tones. An A/B/Y pedal would quickly turn this amp into a nice gigging amp with the possibility of switching between clean, dirty and lead tones. I can't wait to fit this one in a box!

I'll post some pictures later, they are on a different machine..

[David Root]

I'm amazed about the 807s! I always thought that was a finicky tube.

A 5E8 has to be an involving amp if Clapton likes to use two of them on stage! But aren't you way above 5E8 plate voltages? I thought they were about 375Vp or so, haven't checked the schematic lately though.

Correction, I checked, the 5E8A is 400V. Two of them theoretically equals one 5F8A, but I bet two 5E8As sound better! I am a firm believer in two speakers separated by 10 feet or so. With tubes that have good soundstage qualities they make a huge difference I think.

[Cliff Schecht]

I'm at about 410-420V on the B+ (depending on what tubes I put in) and slightly less for the plates, like in the schematic. It's all well within 2% of the schematic voltages.

5F8A is actually a pretty different beast than the 5E8A. LTP PI and TMB tonestack vs. cathodyne and different style TB tonestack in the 5E8A as well as single GZ34 rectifier vs. dual 5U4. I don't know too much about the 5F8A but I think it probably is closer to a Bassman than a 5E8A.

I'm still torn on whether to make this into a combo or a head. If I go head then I can't use 807's and if I go combo, I'm more limited on my speaker choices. I've had great luck with 807's in Fender builds so far but they can oscillate like a mofo unless you add a 10 Ohm resistor wrapped with 10 turns of wire. This is an old ham trick that acts as a low Q choke to prevent RF oscillations. I'm thinking a head would be cooler and more in tune with my current needs.

I love dual cab setups myself and run them whenever possible. Fender tweed era amps have a great soundstage and a lot of harmonic complexity that plays well in mixes with one cab. When you add two cabs, especially two with different characteristics (e.g. I like an open and closed back cab ran together), you can get a very complex and interesting sounds.

[paulster]

I love the 5E8A, both for its thick cleans and for its crunch tone.

I use mine with 5881s through a Vox 2x12 loaded with Celestion Golds and couldn't really think of a better combination.

[Cliff Schecht]

Which channel and input do you prefer Paul? Also what preamp tubes are you running? NOS 5881 or new Tung-Sols?

I think I'm going to make some kind of simple footswitch to choose between inputs because between them there are so many sounds available. I agree about the thick cleans and great crunch. I can't wait to try this circuit in the studio and live, I can tell it's going to be a go to amp.

[paulster]

Which channel and input do you prefer Paul? I think I'm going to make some kind of simple footswitch to choose between inputs because between them there are so many sounds available. I agree about the thick cleans and great crunch. I can't wait to try this circuit in the studio and live, I can tell it's going to be a go to amp.

I only used three preamp tubes and one input jack (for some reason I really don't like lots of input jacks), so I just built one channel but added a bright switch to give me that option back, and also a boost switch to parallel the two triodes to give me the jumpered option that you'd otherwise have (otherwise the second triode input is grounded).

Since I'd lose the additional gain that can be had by leaving the unused input gain at about 2 o'clock on the dial (500K on the pot) I upped the 270K mixer resistor that would otherwise be tied to ground to allow me to get this additional gain too if I wanted it.

I tend to use the boost mode with bright on, or sometimes bright only.

So mine's like the Rocket take on the AC30 but applied to the 5E8A. It's definitely one of my favourite amps for its versatility and sheer tone. If you get a chance to try it with some Celestion-style alnicos I'd really recommend it.

[Phil_S]

I'm still torn on whether to make this into a combo or a head. If I go head then I can't use 807's and if I go combo, I'm more limited on my speaker choices.

Cliff,
Maybe it's just me, but I'd opt for the head over the combo any time. There are two good reasons and I'm sure you already know what they are. The first, and most important is the simple flexibility of having a head that you can plug into a variety of cabinets -- that and the weight issue for moving it around. The second is that with a combo, you'll have to deal with the possible complications from speaker vibration -- a nuisance, if you ask me (which you didn't).

What am I missing about not being able to use the 807's in a head? I think I don't understand where that 10z resistor goes. I'm thinking you can solve the oscillation problem if you really want the 807's.

That's just my 2 cents.
Phil

[Zippy]

5F8A is actually a pretty different beast than the 5E8A. LTP PI and TMB tonestack vs. cathodyne and different style TB tonestack in the 5E8A as well as single GZ34 rectifier vs. dual 5U4. I don't know too much about the 5F8A but I think it probably is closer to a Bassman than a 5E8A.

The 5E8A is a whole lot like the 5F4 Super - with a 10M instead of a 4.7M in the feedback loop around V2 to make it a lil' looser. I didn't realize the 5E8A was what Clapton was using on stage. Thanks!

[paulster]

Clapton's Twins are nothing like standard ones.

They are solid state rectified with 4x output tubes from what I remember.

John Suhr mentioned over at HugeRacksInc broadly how they'd been modified sometime before (since he did them) and the only resemblance to a stock Tweed Twin is the cabinet really!

[Cliff Schecht]

Funny thing is I had no idea Clapton used these amps until after I decided to build this amp and dug deeper in the interweb for more details. As with most of my builds this one was dictated by what I had on hand. I'm not a fan of his modern sound anyways, his noiseless pickups sound pretty lame imo. I think the modified tweed twins are his saving grace as far as not sounding like crap.

Also I got to pop in a fresh set of 6072's in the amp and it sounded much better clean wise. The distortion is still titties but it wouldn't clean up with the 3 unknown strength 12ay7s I had in there. Good thing good ay7s can still be had for cheap!

[David Root]

I didn't know that about Clapton's Twins. They look like 5E8s alright, but that's as far as it goes it seems.

If you're going to use four tubes anyway, why not use two 5F8s? Maybe he likes the cathodyne PI. He does other strange stuff, like trying to get LP tone in a Strat.

[godi]

This amp is on my list of ''next amp to build''. Can someone confirm that the bright and normal channel can not be jump like we can do with a JTM45 or a 5E3 Tweed Deluxe ? And why ?

If I use KT66 instead of the regular 6L6, should I raise the plate voltage on the KT66 ? I was thinking to use a JTM45 output transformer, Is that a silly idea ?

Thank, Jean

[chopstuck]

6072 cheap - where and how ? Fleabay has theirs at $100 a pop but it varies with brand and construction.

What's "cheap" anyway ?

[Cliff Schecht]

Literally yesterday I was paying $4 for 90%+ used stock and 10$ for NOS 12AY7's and 6072's. I bought every one I could find (except Sylvania's), and this guy had literally 20 boxes filled with tubes (and kept telling me a warehouse full of more, including sockets, caps, everything I want for cheap). I also scooped up some I63 Mullard 12AX7's labeled for RCA (neat!), literally handfuls of 807's and 829B's (both like 6L6's), other oddball AX7's I could find, 5U4's, a NOS Genelex KT61 (paid $3!), crap the list really goes on. One guy also had a real pair of gently used Genelex KT88's for $200 that I'm strangely considering and a few NOS Mullard 5AR4's for $50 (let those slide). Last thing I did buy for $20 was an 833a, completely useless but very big and cool tube.. (pic: http://www.tubecollector.org/main/833a.jpg).

I say all of this not so much to brag (ok, a little) but to point out that there are deals out there to be had. Unfortunately you aren't going to find very many deals on eBay or anywhere that sells quantities online, there are just too many other people in the world to compete against with more money and less brains. You have to find the alternate sources, mine is ham radio guys, that stumble across tubes and know what's worth money to a certain point. They don't give two rats asses about audio tubes however and are usually either giving them away or are selling them for *reasonable* prices if they are in the know. Look in your area for ham radio events, anything with a swap meet. There are lots of guys that spend their time just searching out old tubes to sell to others. Not the big name guys you find online, these are small-time guys that make their living selling tubes at hamfests and to a small but steady customer base.

My other big source is buying old dead test equipment that uses tubes I'm looking for. While these are almost inevitably used, many times the tubes test over 80% which is fine for guitar amps IME. A surprising amount of equipment gets retubed and put on shelves for too many years and it goes south. If you find this stuff at swap meets or junk stores, it's usually cheaper to buy the whole piece of equipment than it is to buy the tubes out of the device (they still have to dispose of the old equipment). Just last week I was given a Heatkit HF sweep generator (uses a DC saturated coil to tune, impractical nowadays but cool) that had a Mullard I61 AX7 in there that looks nearly brand new. Also had a few yellow Astrons that where very usable..

[chopstuck]

Rats, I thought you had an accessible source (for me). I have been getting the bulk of my tubes from old Dyna and Sunn gear I bought in the 90's when it was cheap.

Funny I'm also working on a low power tweed twin. I've built so many tweed Supers that I want something different. I'm limited by the Super chassis so only 3 Novals.
The redundant channels just aggrrivate me from a design standpoint.

I have a 394-0-394 PT and a choke the size of a tweed bassman OT.
I have lots of old Organ parts so I may build with a choke input filter to get my B+ down.


Maybe I can trade you some transformers for some tubes ?

Flat rate boxes ship cheap. Just the ticket for transformers and gold bars.