DIY Audio Projects Forum

Very old small amp
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Author:  Wuh [ 28 Jan 2020, 14:08 ]
Post subject:  Very old small amp

Hi all :beerchug:

Some time ago i bought a very small tube amplifier and i want to replace every cap and resistor in it.
Also the tubes i want to replace.I have no experience so my questions might look stupid.I know i can find a lot of information on internet but i need to be 100% sure i do and choose things right.

Let's start with the tubes; There are 2* 30A5-9CN tubes and 1*12AX7 tube.
*Do the 30A5 tubes need to be matched?
*Can i replace somehow the 12AX7 tube with 6H1N-EB? (I have a few 1984 NOS)
The big capacitor next to the transformator is in a round clamp on the board and has 4 connections on top and read this on the side; Suzuki 60.40.30 MFD red yel neg black 150WV

This are the other caps and resistors;

* 50uf 10v
* 2l 100Ω Suzukiohm
* 2L 1.5k Suzukiohm
* Suzuki oil JCP-A 0.002uf 400WV (2 pieces)
* Same as above 0.01uf (2 pieces)
* same as above 0.005uf (2 pieces)
* same as above 0,001Uf (2 pieces)
* Me 630v 103k (2 pieces)
* Suzuki 2M 100K
* Suzuki K 5KΩ
* 01 uf 500 (This is a round flat orange thing,everything else is axial.)

Can someone explain please the above caps and resistors?How to read them,with what can i change them and everything else what's worth mentioning.


* This might be the scematic but i,m not sure.I have 3 tubes and some pictures on internet have 4 tubes.Maybe phono?
* Is it better to replace all the old wiring?
* How can i find out what Ω speakers i need to connect?
* Other things i need to know?

It's a lot of questions but i tried to be as clear as i could.My goal is first to bring it back to it's original state with decent components.If that works and i understand the amplifier better i can maybe try some tweaking.I understand this is not HiFi.I just want to play with a vey simple amp to learn.

Thanks for reading!

Author:  samc [ 29 Jan 2020, 00:16 ]
Post subject:  Re: Very old small amp

Ok, a few comments.

The WV on your capacitors probably stands for "Working Voltage" -- which is what voltage they can consistently handle during operation. I would seek to replace these capacitors with capacitors with an equal to or greater voltage rating and the same capacitance. A simple suggestion would be to avoid carbon composition resistors and electrolytic capacitors where possible, so long as power and voltage ratings are maintained. A word of caution here, the capacitors can hold a charge for a long time so you need to make sure to discharge all the capacitors before touching anything in the amplifier. There are good you tube videos showing experienced technicians doing this. I would watch a few. It's vital that you understand how to do this work safely before attempting it.

The schematic you posted doesn't match the tubes you have. If you shared the model number of your amp, someone might be able to answer the other questions you have more thoughtfully.

Author:  Wuh [ 29 Jan 2020, 07:53 ]
Post subject:  Re: Very old small amp

Thanks for the reply.I go try to find the good values but i can't find much on all other things then the tubes and oil caps.But i'm not in a hurry and will post the basket first here before ordering.The brand and type is palace sa-80s.I made some pictures.I am aware of the danger but for now i don't power it up untill i changed the parts.A variac,isolated transformer and lightbulb are on my list next weeks.I am on budget and in contact with a seller who has some nice monoblock EL84's for sale very cheap.If i can buy them all other things have to wait :D

Author:  Woodo [ 29 Jan 2020, 17:12 ]
Post subject:  Re: Very old small amp

fyi, looked up your amp in dr google and found this german forum thread. unfortunately it is in german but you may be able to translate.

you may have good reason to bring your amp back to life and it definitely looks like a worthy challenge, but you could also consider many of the more recent headphone, phono, preamp, power amp builds in the project pages and in the vacuum tube forum posts of varying complexity that the designer/builders have carefully thought out and for which support is more likely.

Author:  Wuh [ 29 Jan 2020, 19:11 ]
Post subject:  Re: Very old small amp

I found that german forum thread earlier but my german speaking is not very good.Thats where i have that scematic from.I have that in my favourites and i go read that better soon :)
There are many nice projects and i go try out things soon.I have no experience in this and still collecting all kind of tools while i'm on a budget.This amp is complete and some kind of playing (the sound is really bad).To me this looks like a repair i can accomplish without to much trouble.If i am able to get it playing normal again it would be a great motivation for me.
The whole tubeworld is so overwhelming to me at the moment that i feel i have to start with small things so i can learn more easy.I'm also in contact with the seller of the amps at the bottom of this page; It's fully working and with good tubes.That might be a good next step to understanding and play with other/better parts and with the seperate power boxes a good base for future projects too.At the moment it's a lot of money for me as i invested so much in all kind of gear lately.

Author:  gofar99 [ 29 Jan 2020, 21:35 ]
Post subject:  Re: Very old small amp

Hi, It appears to be a budget amp from about 1960. The 30a5 tubes are not common any longer and may be costly the 12AX7 is common. First off I would not replace them unless they are non-functional. The large filter capacitor for sure, all other parts I would wait and see if it powered up. You can easily spend a lot more on the amp than the price of a newer decent one that works. Second and most important these types of amplifiers were often directly powered from the AC mains. Thus the relatively high heater voltages. The tubes would be in series with the AC mains often a 4th tube likely a 35W4 rectifier was in the string. A more common layout is a pair of 35C5s, one 12AX7 and one 35W4. To get around not having the 4th tube there was usually a large power resistor needed to handle the rest of the voltage (120 -72 in your case). For rectification it was often a selenium rectifier.

NOW HEAR THIS. If the unit is directly line powered it is dangerous to use. This forum and all others I know of prohibit discussions of such equipment. Additionally if there is a selenium rectifier (I can't tell from the photos) it can emit toxic vapors if it is overheated. Personally I would go for the EL84 mono blocks. They are almost certainly of reasonable quality...I have never heard an EL84 I didn't like. Save yourself some aggravation in trying to fix the one you have and possibly not kill yourself as well. Good diyers are scarce and we don't want to lose any.

GIO: please correct me if the site policy is different from what I understand.

Good listening

Author:  Wuh [ 29 Jan 2020, 22:50 ]
Post subject:  Re: Very old small amp

I found out the oil caps are most likely ok and very expensive to replace if you can find them.I thought it was only a few $ to replace all parts.The safety issue is a good one.I can do all things right while working on it but if there is danger while operating the amp i can't proceed with it.I put it back together and store it untill i'm absolutely shure i know i am experienced enough to even look at it again.Thanks for the warning.I thought only older gear had this safety issues.
What i learned is to do only more common and modern projects so i can use others experience to avoid hidden surprises.

I found this great source with books and daily new books are added 8-) Enough to read and learn on every level.

Author:  mwhouston [ 31 Jan 2020, 17:46 ]
Post subject:  Re: Very old small amp

I don’t know the tube but it reminds me of a 6T9. The 6T9 has a triode and pentode in the one envelope.

Not the best sounding tube but at one point Spare Time Gizmo was selling kits to make a neat little amp. A PCB was included. The 6T9s got hard to get (though I still have a few) and he stopped making the brds, He sold them as a triode/triode kit but by cutting one track (easy to do with a Stanley knife) you could have a pentode output if your OPT supported it. That is how I made mine with Edcor OPTs with feed back tap.

I’ll put up a link.

Author:  mwhouston [ 31 Jan 2020, 17:48 ]
Post subject:  Re: Very old small amp

The links

Author:  Wuh [ 31 Jan 2020, 23:51 ]
Post subject:  Re: Very old small amp

Hey thank you!

For me as Dutch this is the reference. I am close to order the first parts for little ceasar :P I thought he died or something but then i met him running this site In the first email he invited me to a event where audio lovers come together.For me,and many other Dutch he is sort of a king in tubes.Look on his site what he sells :-) Good solder and good wire.Not much more.What he sells is what he use.I like that.No big webshop with all kind of crap but just a few things he want us to use because of his personal experience.The "modern" webshop on his site is filled with crazy bargains.You have to look closer to find $1000 products at €300 because it is modified (better)
Believe me,this guy is that guy.

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