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Old 4th March 2013, 11:32 AM   #1
Tesla88 is offline Tesla88  Italy
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Default Restore or build from new?

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 Spiders included

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 ---
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Old 4th March 2013, 01:21 PM   #2
indianajo is offline indianajo  United States
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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).
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 4th March 2013 at 01:23 PM.
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Old 4th March 2013, 01:26 PM   #3
DavesNotHere is offline DavesNotHere  United States
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wrap some wood on the outer frame, clean and paint the top and transformers. build a PS separate because of the advantages of split chassis.
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Old 4th March 2013, 01:53 PM   #4
overtheairbroadcast is offline overtheairbroadcast  Canada
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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).
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Old 4th March 2013, 05:55 PM   #5
Tesla88 is offline Tesla88  Italy
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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

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 ) , 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?
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Old 4th March 2013, 05:57 PM   #6
Tesla88 is offline Tesla88  Italy
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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?

Last edited by Tesla88; 4th March 2013 at 05:59 PM.
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Old 4th March 2013, 08:20 PM   #7
trobbins is offline trobbins  Australia
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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.
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Old 7th March 2013, 01:12 PM   #8
Tesla88 is offline Tesla88  Italy
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What about using Dynaco ST35 schematic? an ultralinear PP with EL84 ...

On Week End i will trace actual circuit in my spare time...
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Old 7th March 2013, 05:49 PM   #9
rmyauck is offline rmyauck  Canada
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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.



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.


Last edited by rmyauck; 7th March 2013 at 06:11 PM.
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Old 12th March 2013, 03:42 PM   #10
Tesla88 is offline Tesla88  Italy
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Here's the traced schematic of output section , not sure about phase splitter - drive section , seems weird , value of resistors at least seems strange

Click the image to open in full size.

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