• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Restore or build from new?

Tesla88

Member
2011-12-21 8:37 am
Italy
Hi ,
yesterday i purchased an Old amplifier from a Juke Box(Electro Kicker) for a good price :

eBay Italia - Aste online e shopping a prezzo fisso

Don't have a schematic and don't know nothing about tubes used and their position. Anyway output tubes should be EL84 in PP , and OT got UL taps and dual impedance secondary.The driver should be a double triode like ECC83 or similar , didn't have enough time to discover if input tube wiring is for pentode like EF86 or a double triode.

Power transformer is missing , i was thinking to supply the unit from bench supply (after replacing at least the 3 big caps ) to try if works , but found some bad resistors (out of range ) ...
Chassis isn't rusty or ruined , just too dirty :D Spiders included :p

So i was thinking at 2 possibile way :

1- Build it from zero , on a new chassis with enough space for the power transformer (better looking but more expensive , maybe)

2- Place the Power Transformer beetween the power tubes , removing the driver tubes sockets , remove the Tone and Balance controls to accomodate sockets for driver ... anyway at present there are 2 tubes for each channel (front end and phase splitter) maybe could be reduced .

What would you suggest ? Start from zero or revamping?

Also , i will trace at least the output and phase splitter configuration and feedback path for sure ... I think is better not to modify the power section , but is there any good sounding schematic for a EL84 PP
Ultralinear amp ?

--- It seems my GU29 project is going into stand by mode ---
 
If the chassis isn't rusty, cleaning with ethylene glycol monobutyl ether solution (non-chorine bathroom cleanser) a paper towel a screwdriver & a pick can make it look a lot better. I even had to so clean my chrome plated "legendary" ST70 amp 2 years ago. The pick is for around the components on the PWB.
Punching and drilling holes in chassis is boring and makes a mess. I would try to reuse this one. The bare chassis and the tooling are expensive, also.
If I bought a new power transformer from edcor, hammond manuf, or triodeelectronics.com, I would buy one with enough power to run EL34's, in case you decide to upgrade at some point. EL84's are good for about 13 w Push pull, which is not very much even with efficient speakers.
Your "hifi" output transformers may prove to be less than high fidelity. Ability to pass high frequencies has to do with what kind of iron and how thin the laminations are, perhaps other design details. I would replace obvious out of tolerance resistors, upgrade the plate and high voltage grid capacitors to plastic film, replace all electrolytic caps, and do a frequency sweep to see how linear it is. If you end up not liking the response, you can put in ST70 quality output transformers or better, punch holes for octal sockets, buy an EL34 quad and a 5AR4 rectifier, and march up to at least the second level of sound. (My stock dynaco ST70 is inferior in sound to my djoffe bias modified dynaco ST120 amp).
 
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How much experience do you have with building tube amps? You have done this before? This is your first time for tube amps? Or have you experienced building amps before (chip amps, Class A amps, Class AB amps, etc...)? You are comfortable with 300 Volts and above? And how much do you want to spend?

What do you want to build? A SE amp or a PP amp? How much power do you need for your speakers? What kind of speakers? What kind music do you play? What did you eat for breakfast this morning? All very important questions (my Italian friends all say that the breakfast espresso is especially good standing up in a cafe while heading to work with quality music and quality headphones on).

Since you have the chassis and the output transformers (and possibly a choke) would I be so bold to say this is a good candidate for an RH84 build possibly? Need good sensitive speakers, a transformer, tubes, and the rest is almost right there in front of you. Or some type of EL84 amp in UL (could mean tweeking the RH84 design but with a connection to a UL tap in the soon to be there new power transformer).
 

Tesla88

Member
2011-12-21 8:37 am
Italy
Thanks guys for the "cleaning - restoring" info, i know this could be a "not so hi fi" amp , but for 30€ it deserves a try , no? At worst i can sell it as not working :p

No need of more power , i've got another amp with EL34 PP but I hope the OT are good quality , but who know? The UL taps made me think they are quite HiFi

I really like the idea of using wood wrapped around existing chassis , but i won't go for separate PSU , really i don't like it , sorry :(

Next days i will trace the actual circuit to see if keep something or redesign the circuit.
Do you suggest to keep the driver/feedback/output stage? I think the front design for really low signals (maybe phono) due to the number of stages , a schematic is needed but it take some time to be traced entirely , so maybe is better to keep only the power stage design.

To overtheairbroadcast:
Thanks for care , but i'm experienced in high voltages , and have built several amps solid state and tubes ! I start repairing CRT television at 11Years (thanks my uncle who worked for many electronic companies like Geloso and also IBM when Computer where room sized :p ) , my first tube amp was a Semprini ST280 at age of 16 :)

Little OffTopic To indianajo :
I've another amp with EL34 in UL PP half build , i'm only choosing what driver / front end configuration use (Dynaco vs Mulard vs Williamson ) Any suggest?
 

Tesla88

Member
2011-12-21 8:37 am
Italy
I've also found an easy way to get a new chassis and i've got all tools for the mechanical work...but is always boring ;)

Just a question , how much power do I need to supply the amp ?
I've got a 300V / 200mA - 6.3V /6A rated transofmer , I think is enough , am i wrong?
 
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Sounds to me like you would benefit from doing a circuit schematic and understanding in your own mind all the circuitry and its operation, and what valve types would have been used in the various positions, and then work through all the power needs of the amp. This may require you to prepare a 'design' document where you collate all the information you can find on the amp. If you can't describe the amp, then there is more to get to know about amps in general - if that is what you like about amps :)

You can go a long way to understanding the existing design, and appreciating what may be some options for modifications, without fully restoring the existing amp. The main thing to test are the output transformers, and if they aren't known parts then you need to deduce the winding impedances.

It sounds like your power transformer would be fine, but you need to at least settle on the likely valves, and add up their current requirements.
 
Another ST-35 circuit to look at is the diytube one as it is the same circuit , but uses separate 12AU7 and 12AX7 types for tube rolling. The7247 as you may know is made of those 2 tubes in one bottle. Many like to replace the 12AU7 with 12HB7 or 6GC7/6FQ7 types. There are lower gain types that will replace the 12AX7 also. Schematic is in the manual download.

http://www.diytube.com/phpBB2/viewforum.php?f=1

http://www.tronola.com/

Dave Gillespie's EFB fixed bias mod also found on that site and the Tranola website is also well worth doing also for very low $ as it extends output tube life, improves sonics, and adds a few watts/CH.

You can buy PCB boards also on the diytube site and Triode Electronics sells them with parts or whole amp kits.

I don't think you will regret building that PP EL84 amp with the mentioned EFB mod which you can add later. The EL84 tube are well like sonically by most in PP and you can't beat the price for them. One just needs fairly efficient speakers!

Dave Gillespie is also coming out with an EFB PS board within this month if you want all PCB's.

Randy
 
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Tesla88

Member
2011-12-21 8:37 am
Italy
Anyway xformers tested are good !
What do you think about the original design , expecially the phase splitter driver section .
Is that a good design or is better to start from a better one ? Can you please suggest best schematic for a UL PP?
Also , are the 2.2 K screen resistors a standard value ? I've always seen lower values such a 100-220 ohm...
 

Tesla88

Member
2011-12-21 8:37 am
Italy
I'm only in trouble with a 72K on phase splitter cathode and also plate load is high , maybe i've just never seen this kind of PI.
When yuo say " you haven't seen some of the circuit sctions before" do you mean there's a circuit section on this forum? or are you talking about circuit sections from various amps ?
Anyway i'm still want to know what would be the best circuit for a EL84 UL Push Pull..
 
Sorry for confusion - I meant circuit sections in various amps. That basic long-tailed pair PI was common - I presume as it had very few parts, especially when dc coupled from previous stage. The balance is improved with tail 'length', so 72k is reasonably 'long', and as its dc coupled, it has to match the operating point of the input stage. DC coupling makes it easier to apply feedback - but that may be your first modification to try.

Some just want an amp to mirror the sound character of a commercial design, and so that may provide incentive to change the design - but that approach can fail miserably unless you change all the circuit stages and parts etc etc.

One approach is to get the amp going as is, and then work through identifying where it may be deficient. For example you may feel the PI should be better with a larger headroom capability, but in practise, your amp may never get the PI in to overload. Or maybe the overload of that particular PI design is what will set the amp apart from the rat pack.

If a stage starts distorting before any other, and the sound isn't pleasant, then there are often ways to soften the distortion before ripping the design apart and changing completely to another design.
 

Tesla88

Member
2011-12-21 8:37 am
Italy
OK , thanks for the replies! I measured the original circuit and most of components are out of value , plus i really don't like the chassi , so i'm going to buil from new keeping the output transformer .I will soon post some pic of layout.
I'm going to use 3 tubes per channel , 2 obvious are the EL84 and third is the input stage/PI tube.

Candidate are :

- http://www.diyaudio.com/forums/tube...-circuit-referenced-tubecad-journal-site.html

Really like the design and well built ! With CCS and Led at input to be tested :)

- Baby Huey PP EL84 amplifier - diyAudio

I like the Idea of having a balanced input stage, i have XLR sources sometimes...but need a CCS at input :(

- Bevois Valley from Morgan Jones

Is not that different from the Broskie's amp i think ...

I don't want to build the Red Light District , i prefer a CCS bias for the output stages.

What you suggest ?
 

Tesla88

Member
2011-12-21 8:37 am
Italy
Hi , i've recently built the AMP based on EL84 ... is just a benchtest on a piece of wood at time but i'll make the hard work ASAP.

I used this tubecad design ,using 5814A in place of 6N1P , tried E88CC but still loving the 5814A ...
PS-4 & PS-5 Tube Power Supplies

My question is about power , i used output transformers grabbed from an old Kicker amplifier (jukebox) they are 8K plate to plate with UL taps.

I'll post as soon scope traces showing good linearity from 20Hz to 20KHz , square wave response also good until 20KHz

My question is about power , i'll started with B+ about 320Vdc , output was [email protected] , so 8W sinus power ...mmm
So I increased B+ until 350-360Vdc , this give about [email protected] meaning 12,5W sinus power.

How the hell Dynaco ST35 can give 17W per channel ? Is an EL84 UL PP design , i can't find the B+ voltage on the Dynaco but i won't go over 350V with modern EL84...
 
You wired it up in UL mode? Schem shows triode mode. You may need some Neg. Global feedback for anything , but triode mode.

I think the Dynaco ST-35 B+ might be 360V if I remember reading that correctly somewhere. It is hard on output tubes tubes so Dave Gillespie came up with a nice simple Fixed Bias which allows you to bring the dissipation down so any EL84/6BQ5 can be used & last. It also results in 2-3 watts more power /CH and better sonics. It can be used on any easy to drive output tube like the 7591 types also.

It's called the Fixed Bias EFB mod:

Lot's of info on the diytube site also under ST-35 etc.

The Tronola site has the mod:

Dave's Lab
 

Tesla88

Member
2011-12-21 8:37 am
Italy
Yes , i wired in UL mode , simple connecting screens to UL taps by 100 ohm series resistors.
Tried different biasing methods , cathode resistors , CCS , fixed bias , no way to go over 12W with 350 Vdc on plates , tubes biased at 40mA each
Ripple was about 5V pk-pk above 350V at full load.
The power is enough and sounds good but i'm triyng to understand why i don't get 15-17W
I'm using "General" Tubes (matched) ...but same power with NOS Siemens
Tried Pentode mode giving more power but sound worse :D

I will try with different load (to find the optimal load)

It's time to take a look at trasformer ratio...
 

Parafeed813

Member
2012-08-15 8:24 am
At what % are the UL taps?
50% would give less power than for instance 20%.
I haven't recently used or measured my old EL84 amp, but I think it produces around 12W, running @ 340V, 8k5 opt with 40% UL taps and a little cathode feedback.
17W is pentode mode from the datasheets (300V, 8k).