EL34 on a PCB?

a DIY, modified Mullard 5-20 monoblock design

Biasing Tweak

Postby EWBrown » Mon Aug 18, 2003 8:24 am

This little "tweak" is for Dyna ST-70s, but should work well on any amp with Fixed Bias.

A quick and very easy mod for the St 70 to tighten up the bass and damping factor is to simply add a 10 uF cap across the bias resistor that goes from ground to the ground leg of the bias pots. If you really want to get tight bass and two more 10 uF caps across the wiper leg of each bias pot to ground.

Of course, newer designs with SS rectifiers and more capacitance also help enormously in the headroom and bass response area.

I was looking over the "guts" of a VTL MB450 monoblock this weekend, it has mongo huge MIT caps, 2 x 3200 uF/ 450VDC, and solid state diodes, I don't immediately recall the part number, these had spherical ceramic bodies and very thick leads. The tube complement in this "monster"
is 8 6550s, a 12AT7 and 6350 (like a 12BH7 on steroids) for input and phase inverter. The power and input trannies are enormous. The amp weighs about 80 pounds.

/ed B in NH
Last edited by EWBrown on Tue Oct 28, 2003 12:16 pm, edited 1 time in total.
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EICO website

Postby EWBrown » Wed Oct 01, 2003 1:04 pm

Found this whilst surfing... (OK, while goofin' off at work) :lol:

http://users.rcn.com/fiddler.interport/eico.htm

There is a zipped gif schematic for the HF-87

BTW, this is one of those strange cases when a PKZipped file is
larger than the original .gif file.

If one converts it into a jpeg file, it balloons to about 780K. Ouch.
/ed B in NH
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Re: EICO website

Postby Shannon Parks » Thu Oct 02, 2003 6:53 am

EWBrown wrote:Found this whilst surfing... (OK, while goofin' off at work) :lol:

http://users.rcn.com/fiddler.interport/eico.htm

There is a zipped gif schematic for the HF-87


Hehe, I've been here many times. Interestingly enough, the Eiclone very well named, as you can look at the HF-35, HF-50, HF-60, HF-87 and HF-89 models and just see small changes - the Eiclone is similar to all of them.
Note the EF86 used as the voltage amplifier versus the 12AX7 on the 87/89 (ie the stereo units). I've kind of reversed it. I wonder what the price difference on these two tubes was 40 years ago?

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EF86

Postby EWBrown » Thu Oct 02, 2003 11:02 am

From what I could gather earlier, the EF86 is similar to, but better than, the pentode section of the 7199 used in the Dynaco ST-70 and SCA-35.
The "Uncle Ned" ST-70 replacement driver board uses 2 EF87s, and either a 12AU7 or 12BH7 as the phase inverter, rather than 2 7199s.
The EF86s are usually a tad pricey, but I did find a source for then on E-bay, from the Boston area. He sells the "National" branded Russian made ones, about $40 for a sleeve of 5, which is less than most other sources.
The pentodes have a gain of 250-300 versus 100 for the 12AX7, this was quite noticeable when I was in process of modding the SCA-35 that I now use in the lab at work. First step was to bypass all the tone controls and "filters" which only served to mangle and mutilate the audio signal.
next step was to disconnect the preamp section altogether, and just go
direct to the volume and balance controls straight into the PA boards. Better, but the 7199s still didn't sound right. Next was to purchase two
replacement ST-35 PCBs and stuff them with left over "mouser" parts.
This improved audio quality nicely. I had considered trying to stuff a "blue board" in there, but it would have required more mechanical re-work than I was willing (or able) to do. Last remaining step will be to replace the original silicon diodes with the UF4007s, and put in more filter capacitance - I plan on adding two or 3 2X32 uF 450V J/J caps.

Back to the Eiclone: I notice in the original HF87 schematic, they use two different value resistors in the plate loads of the 6SN7 phase splitter, 33K and 28.750K, similar to how Dynaco used 33K and 27K in the ST-35 (and SCA-35) "cathodyne" phase splitter.

I like the idea of making the Eiclone configurable, I just happen to have two pairs of 6B4Gs I obtained recently, and a couple Chinese made Valve
Art 300Bs.

The next "blue sky" project will be SE amps using 211As, 845s or WE 276s, I am slowly gathering the parts for this, and am still in the process of deciding on a finalized circuit. Gotta have something to keep myself from going crazy with cabin fever during the endless NH winters...

/ed B in NH
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phase splitter load resistors

Postby Shannon Parks » Fri Oct 03, 2003 6:54 am

EWBrown wrote:Back to the Eiclone: I notice in the original HF87 schematic, they use two different value resistors in the plate loads of the 6SN7 phase splitter, 33K and 28.750K, similar to how Dynaco used 33K and 27K in the ST-35 (and SCA-35) "cathodyne" phase splitter.


I actually did real testing here, as there is no formula and many manufacturers approached this differently. Even Eico used a 27K and 33K in the monoblock amps I mentioned, but used the 28.75K and 33K in the stereo versions. Some manufacturers used the same values (eg 33K and 33K). Did it have something (everything?) to do with the OEM phase splitter that shipped with the unit? Others threw a pot in there so the user could set the optimum balance between the phase outputs. I originally just threw a 30K and 33K in there (due to being standard values - there is no 28.75K) but decided to actually <test> this several weeks ago. :munky2:

The 30K and 33K were a perfect match! Even dropping the first resistor to 28.75K (yeah I was using a pot) made the outputs unbalanced and 27K was very bad. Now will this vary from tube to tube (6SN7's in question)? I dunno. But I certainly will test ith a slew of different ones, both new and old.

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Speaking of EF86s

Postby EWBrown » Fri Oct 03, 2003 1:07 pm

This is where I got mine a few months ago, he almost always has them available

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 2561630030

/ed B in NH (if it doesn't freakin' rain AGAIN :angry5: :banghead: :badairday: this weekend, I'll go parts hunting at the "Hosstraders" hamfest).
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More parts

Postby EWBrown » Mon Oct 06, 2003 5:58 am

The rain held off until about noon, so I did some bargain parts hunting at the "Hosstraders". 10 NIB 10K Clarostat pots for $5, 10 470 uF 400VDC caps (pulled from switching power supplies) for $6, and some of the
3-terminal strips I needed for the thirdl DIY-35 amp.

The tube sellers have wised up as to what tubes are good audio tubes and have priced them accordingly, so $1 EL34s are a thing of the past - about 10 years ago... Saw, but didn't buy a very clean H H Scott integrated amp
for $250, I don't recall the model number, but it used the hard to find 7591A tubes. About 35 w/ch. The downpours started around noon, so I bailed out while still dry.

If anyone is in the Northeast area, the NEARC (New England Antique Radio Club) has a swap meet on Saturday, Oct 11 at St. Stanislaus hall on Pine Street in Nashua NH. I usually go for parts hunting, can always find deals on tube sockets, caps, power resistors, etc.

/ed B in NH
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Tube rolling

Postby EWBrown » Wed Oct 22, 2003 5:21 am

I started out with some "generic" Russian made (Reflektor) 6P14P-EV
6BQ5 equivalents, the amp soundsd quite good, then switched to JJs
for all six tubes, and it sounded a lot better, the 6P14Ps were fairly inexpensive, something like 16 for $50 at the time, unmatched, untested and in generic "ProComm" boxes. I use these as my "sacrificial" tubes in the unlikely event that something goes wrong on initial launch.The 4 JJ EL84s are a matched quad.
I also tried a matched quad of Sovtek "grade B" tubes, they were in between the Reflektors and the JJs. The Sovteks are "guitar amp" grade according to the markings on the box, they do have a better "audiophile" grade available. The next completed amp will receive a NOS set of real Dynaco tubes that I have had salted away for a special occasion.

The Sovtek "guitar" tubes are now residing in a heavily-modified SCA-35 turned into an almost-ST-35 that I use for background music in the lab here at work.

/ed B in NH
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Postby nineno » Wed Oct 22, 2003 4:26 pm

So when do the rest of us poor slobs get a chance to hack one of these together?

My college graduation and christmas are both coming up pretty quickly. I'll need to milk them both for as much as i can. HAHA.

Of course, if the boards hit the auction block before the afore-mentioned dates I'll just have to pawn my text books back early to pick the boards and components up. :idea:

cheers,
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eiclones vaporware?

Postby Shannon Parks » Wed Oct 22, 2003 5:50 pm

nineno wrote:So when do the rest of us poor slobs get a chance to hack one of these together?


Better late than never, eh? Let's just say Christmas is creeping up fast!
:x-mas:
I just ordered twenty boards this morning. Thermion is helping me test so that will definately help out some. He's going to build a pair of fixed bias ones, like HF-60s (with 4.3k trannies so he can slip KT88's in there if he feels like it). He needs thump to his bass. I'm probably going to make one with cathode biasing, ala half an HF-87, and one using 6B4-G's. I've got about four different power trannies on hand and even a few tube rectifiers (tests with 5AR4s and 5U4s) to boot - so we'll be doing many combos, swapping them into a test bed I'll fab as soon as I get my Greenlee Slug Busters!

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5R4s ?

Postby EWBrown » Fri Oct 31, 2003 9:34 am

Have you tried 5R4s as rectifier tubes? THese seem to be fairly available and cheap, Angela says they make good 5U4 substitutes. These come in two "flavors", one is standard "fat bottle" like 5U4, the other is a ruggedized mil-spec version, in a taller flat top "bottle" and heavy duty "potato masher" base, which would require more room on a tight chassis just to fit. The 5R4s do have more voltage drop than do 5AR4/GZ34s, but then I have a couple of HWT 800VCT 200mA power trannies looking for a good place to hang out :lol:

I also have one HWT 60W 6.6K "clone" of a Hammond 1650P that I found on E-bay, for a decent low price, I'll have to "lurk" for another that is not for sale by HWT, as they take sooooooooo loooooong.... The seller of this particular OPT had to wait "only" 7 months for it to arrive.

I'll have to pre-test the HWT power iron to make sure that they don't buzzzzzz too loud :? before committing them to this project.

/ed B in NH
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Re: 5R4s ?

Postby Shannon Parks » Sun Nov 02, 2003 6:18 am

EWBrown wrote:Have you tried 5R4s as rectifier tubes?


Are these the silver-based crazy/cool (think 50's B-movie spaceship) looking rectifiers that Ned sells? I did look at them, and think that they would be fine. But with 5AR4's being so solid nowadays (from basically every manufacturer, especially Sovtek), that will probably be the preferred rectifier. I'll test with the 5U4's too as many with have a bunch of these laying around. All the rectifier testing is basically making sure that voltage drop resistors in the PS are properly sized.

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So, where do we sign up to order a set?

Postby gogzhad » Sun Nov 02, 2003 10:48 am

Are you taking pre-orders for the Eiclone? It would make a great Christmas gift for my wife...hopefully it will go over better than last year's worm-drive skilsaw...
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