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Audio & Hi-Fi Tips on building an amplifier.

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slidertogrid
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Has anyone built any audio amplifiers? I would be interested to hear any experiences or pointers or tips anyone has to share.

My mate is a keen guitarist and over the years he has had a variety of Guitar amps, he has said for a long time that he would like to build his own, just a practice amp but along the lines of a vintage valve design. Spurred on by a recent article in the BVWS bulletin I offered to help him build one using a ECC83 and a EL84.  It was to be built using parts I had 'laying around'.  I found a suitable circuit diagram and rounded up the parts. The Chassis was the bottom cover from a Pye record player amp which wasn't big enough to hold the mains transformer so we decided that would be located separately. 

I was careful to plan the layout especially around input - volume control and ECC83 base to reduce the chance of stray pickup. We wired the heater circuit first, tested that and then completed the H.T supply and EL84 stage. That stage was then tested and was found to work so we continued to complete the rest. 

First test revealed that it worked well but was a bit 'fuzzy', the tone control also affected the volume. We rechecked the build against the diagram and found a mistake in the wiring to the controls, correcting this sorted the tone control problem but there was still a bit of background noise. Eventually we found the heater centre tap that should go to chassis was high resistance, intermittently going open circuit. cleaning the enamelled wire on the transformer and resoldering finally cured the problem.

We then realised it had took us the best part of 5 hours to build it and get it working and it was time for dinner! My mate, Andy was very pleased with the amplifier and it's 'sound' It is running on an old speaker on a baffle that I saved from a scrap Philips radio.  Andy has taken the amplifier home and is going to build a cabinet for the chassis and speaker but I want to build a neater version on a proper chassis with maybe a slightly neater layout, this one is really just a lash up built from scrap... 

It is however the first amplifier I have built from scratch and I am pleased with it. 

I would be interested in any hint or tips for the 'Mark II'!

Rich

20240121 165455
20240121 165522
20240121 174506
20240121 165506

 

 

 
Posted : 21/01/2024 11:53 pm
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irob2345
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Tube Depot in the US (www.tubedepot.com) have a range of replica amp kitsets - my son Adam and I built one some years back. The kit came complete with vintage look vinyl covered case. Uses two EL84s in class AB1.

A satisfying build with well written instructions. Final result looks and sounds great, a little expensive though. Complements his Gibson replica well.

 
Posted : 22/01/2024 12:26 am
Doz
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My daughter plays and sings in a punk band. She ended up with this ...

 

https://andydoz.blogspot.com/2016/04/junque-box-boutique-amplifier.html

 

 

 
Posted : 22/01/2024 8:04 am
slidertogrid
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@irob2345 Thanks, I will have a look it may be that a kit would be the way to go...I will see what Andy want's to spend....

@doz That is an incredible build! way beyond my skills! Very interesting to see what is achievable. 

For now I am going to round up another set of parts for the next one and then Andy will make a chassis. I may suggest a second chassis for the power supply. He wants the controls to be remote to the chassis on a separate panel.  

 
Posted : 22/01/2024 12:43 pm
Doz
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Till Eulenspiegel
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Circuit diagram and parts list for the Cooper-Smith "Bantam" amplifier.

Cooper Smith Amplifier
Cooper Smith Amplifier Parts List

This amplifier appeared in the September 1960 Radio Constructor magazine.  The October edition describes the construction of the amplifier.

I'm considering using this amplifier in a Dansette "Conquest" record player.

Till Eulenspiegel.

 
Posted : 22/01/2024 3:14 pm
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slidertogrid
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@till Thanks that is an interesting diagram using an ECF80 instead of the usual ECC83. It looks like it will have ample gain. You will have the best sounding Dansette ever! 

I saw a Dansette major that had been modified, the EL84 had a ECC83 preamp stage added to it so a lower output cartridge could be used. It had miles too much gain as both triodes had been used. I modified it slightly using just one triode and replacing the valve with an ECC81, that calmed it down a treat and it worked well.

 
Posted : 22/01/2024 7:30 pm
Till Eulenspiegel
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As a substitute for the EL84 the video pentode 6CH6 or EL821 can be considered. The two valves are not directly pin for pin compatible. CV number is 2127. 

From the Radiomuseum:  CV 2127, Tube CV2127; Röhre CV 2127 ID27969, Vacuum Pentode | Radiomuseum.org

The Dansette record player has a simple amplifier which employs a 6AT6 triode, a 6AQ5 output pentode and a tiny output transformer. Also very simple bass and treble controls. The Cooper-Smith Bantam has a much better tone control network.

Till Eulenspiegel.

 
Posted : 22/01/2024 11:07 pm
Cathovisor
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@till That's an unusual amplifier unit for one of Dansette's record players - it was normally used in their radiograms along with a separate chassis for the radio receiver?

 
Posted : 23/01/2024 9:32 am
slidertogrid
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Cathovisor  That amp was used as you say in the radiogram, But I have seen it used on its own in the Conquest occasionally instead of the usual ECL82 version.  A bit of a rat's nest with the controls screwed to a bit of metal and the mains transformer on the floor of the cabinet. This amp must have run concurrent to the usual amp as an alternative for some reason as I have seen very early and late model Conquests with the ECL82 amp. 

As an aside I was tempted to convert or rebuild a Dansette Major amp from EL84 to ECL82 as an experiment to see what effect it had on the gain when all the debate was going on about adding a solid state pre-amp to single stage record players on the other forum. I thought it would be easier, keep the sound 'toobe' and be neater than adding bits of veroboard to the record player. It fell of deaf ears though and I didn't bother in the end. I do still have a amp from a scrap Major somewhere so I could give it a try at some point. 

 
Posted : 23/01/2024 10:27 am
Till Eulenspiegel
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Hi Mike, I remember Dansette made a table radiogram which employed the 6AT6 and 6AQ5 amplifier and a separate radio unit which used an ECH81 and EBF89. Was it model RG30?

An alternative to building a completely new amplifier would be to retain the original amplifier PCB, replace the 6AT6 with a B7G base pentode with similar characteristics as the ECF80. Incorporate the Cooper-Smith tone control circuit and add a pre-amplifier to compensate for the signal loss caused by the tone circuits. The pre-amplifier can be the 6AV6/EBC91 high mu triode. Characteristics are similar to the ECC83/12AX7.

From the Radiomuseum: 6AV6, Tube 6AV6; Röhre 6AV6 ID1449, Double Diode-Triode | Radiomuseum.org  

The 6AQ5 is electrically similar to the 6V6 and thus is not as sensitive as the EL84, gm is 4.1mA/V but the 6V6 gave good service in many radios and grams.  

Till Eulenspiegel.

 
Posted : 23/01/2024 5:05 pm
Cathovisor
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Posted by: @till

Hi Mike, I remember Dansette made a table radiogram which employed the 6AT6 and 6AQ5 amplifier and a separate radio unit which used an ECH81 and EBF89. Was it model RG30?

There was indeed an RG30 David, but in fact there were two models - the RG30 and the RG31. The RG31 was in production long enough to go from a UA14 to a UA16 deck and have those three phono sockets round the back for tape, loudspeaker and add-on stereo. I have a couple of RG31s in my collection.

 

 
Posted : 23/01/2024 5:29 pm
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