Cheap Forum Amp by Captainbeefheart

[LTusler]

Could there be an option to build it ourselves?  A schematic and some good pictures to download for instructions.

[captainbeefheart]

2 hours ago, LTusler said:

Could there be an option to build it ourselves?  A schematic and some good pictures to download for instructions.

 

Yes I encourage anyone with the means to join in of the fun and build one themselves. Will provide schematic, bill of materials, layout etc...

[MEH Synergy]

@captainbeefheart I think I would like one.   I think you still have my number... I don't have yours as I got a new phone. 

[captainbeefheart]

8 minutes ago, Westcoastdrums said:

I think I would like one.   I think you still have my number... I don't have yours as I got a new phone. 

 

I should have yours still in my phone, I'll shoot you a text and you an add me to your contacts in the new phone.

[Curious_George]

25 minutes ago, mike stehr said:

The Edcor OPTs you mention are push-pull.

I had thought the circuit bandied about was going to be some sort of a single-ended pentode amp.

An 8K version might make a for a good 6BQ5 PP amplifier.

 

It seems the pondering has gone the way of SE OTL...I dunno...

 

Opps, you are right. I thought it was PP, but a decent SE xformer will be at least the cost of the PP.

[captainbeefheart]

1 hour ago, Curious_George said:

A quick add up of nominal parts;

 

Output Transformer: Edcor GXPP15 (5K/15W) - $67.52 x 2 = $135.04

KT 88 Output Tubes: Various ones for $50 each - $50 x 4 = $200

Power Transformer: A generic one would be @ $$75

 

We are already at $410USD.

 

If we used NOS tubes from the 6L6 family or a popular pentode family that could bring the cost down significantly. 

 

15 watt 3k:8 output transformers are $30 each, $60

Power transformer is $48

The first couple of amps will come with GE 6550's as I already have them but two KT88's at $50 each is $100

 

We are at $208

 

That is if we go this route. I have  3k:8 Transcendar transformers here that I can breadboard with.

[captainbeefheart]

4 hours ago, Sam S. said:

Curious—you mentioned the carver 275 and tube cube threads. Would it be appropriate (or not appropriate) to mention the Reisong A10 (6w), $399 or A12 (also 6w), $499? Seems like these offerings hit that “affordable” range and are recommended by many. Just asking’.

 

Because like many cheap Chinese tube amps they just do not perform well at all. With 1 watt it's already over 7% THD!! Where it measured 1% THD was only at 40mW of power, pretty terrible if you ask me. For comparison the circuit I am working on reaches 1% THD at 12.5 watts which is pretty good for a SE amplifier.

 

https://www.audiosciencereview.com/forum/index.php?threads/reisong-a10-el34-hi-fi-audio-stereo-tube-amplifier-single-end-class-a-review.19972/

[Curious_George]

1 hour ago, captainbeefheart said:

That is if we go this route. I have  3k:8 Transcendar transformers here that I can breadboard with.

Is Transcendar still winding x-formers? I mean as in new orders?

[captainbeefheart]

17 minutes ago, Curious_George said:

Is Transcendar still winding x-formers? I mean as in new orders?

 

I don't think so. I had to beg Gerry to wind me one last pair of 3k:8 10 watt SE output transformers and that was like 6-7 years ago. I gutted an amplifier I made that had them in it and now they are just sitting around not being used.

[Curious_George]

2 minutes ago, captainbeefheart said:

 

I don't think so. I had to beg Gerry to wind me one last pair of 3k:8 10 watt SE output transformers and that was like 6-7 years ago. I gutted an amplifier I made that had them in it and now they are just sitting around not being used.

I never bought or used any of his products, but I heard they were a tremendous value and good sounding iron.

[captainbeefheart]

Another option that I was doing for people for a little while was they purchase a Chinese tube amp kit, not assembled and have it shipped to me. I then wire up my own circuit for an additional $100. They end up with a really good sounding and performing tube amp for around $350. I know people think the transformers they send are garbage but they really are not. I got great bandwidth, low distortion and good stability quite easily with them.

 

https://www.ebay.com/itm/124915044864?hash=item1d15845200:g:u7YAAOSw-vlVi40P

 

 

[captainbeefheart]

1 minute ago, Curious_George said:

I never bought or used any of his products, but I heard they were a tremendous value and good sounding iron.

 

Yes tremendous performance for a great price. He gets really high primary inductance with his transformers with a low amount of inter-winding capacitance. Without any feedback I could get flat output down to 20Hz and up to 20kHz, add in some feedback and they were easy to keep stable had really good wide band performance. They measured better than Edcor and Hammond. I never really cared for Hammond's SE output transformers, their push pull are pretty good but expensive and I can get the same performance with Edcor for less money. Hammond is right off the shelf though which is nice but you pay for that luxury.

[Curious_George]

27 minutes ago, captainbeefheart said:

Another option that I was doing for people for a little while was they purchase a Chinese tube amp kit, not assembled and have it shipped to me. I then wire up my own circuit for an additional $100. They end up with a really good sounding and performing tube amp for around $350. I know people think the transformers they send are garbage but they really are not. I got great bandwidth, low distortion and good stability quite easily with them.

 

https://www.ebay.com/itm/124915044864?hash=item1d15845200:g:u7YAAOSw-vlVi40P

 

 

 

This was exactly the concept when I bought this amp a few years ago. The EL34 is now one of my favorite amps. I wish I knew where the OPT's were from, I'd buy them out!

 

[captainbeefheart]

9 minutes ago, Curious_George said:

This was exactly the concept when I bought this amp a few years ago. The EL34 is now one of my favorite amps. I wish I knew where the OPT's were from, I'd buy them out!

 

Very cool!!

 

I was pretty shocked at how well the transformers performed also. This is why the kit is a pretty good deal, you have all the iron, tubes, chassis and some parts if you deem they are decent quality all for under $200. Just roll your own circuit in it or improve upon it at the very least and you can make a really nice sounding and good performing entry level tube amplifier.

 

I am sure there is someone out there that made a business of doing this. Purchase the kits, possibly in bulk for a cheaper rate, say 5 of them for $500 or whatever and then do them up right and sell them for $400 a pop. That's a $1500 turnover without much overhead and probably not much time once you got your design down, wiring one up may only take a few hours. Monday through Friday finish one amp per day, that's all soldering and assembly plus testing. Even if you accounted for another $100 in parts/materials per week that's a $1400 profit. Not bad for a weeks work of building simple amps which for some of us is actually fun to do. I may have just found my new side  hustle

[LTusler]

18 hours ago, captainbeefheart said:

 

Yes I encourage anyone with the means to join in of the fun and build one themselves. Will provide schematic, bill of materials, layout etc...

To clarify my ask, I am hoping for a kit along the lines of the VTA ST-70 type kit.

 

Also, any chance of proper balanced inputs?  I would need 2 stereo amps or 4 mono blocks to power my Jubilees.

 

Thanks for all you do on this forum!

[Curious_George]

19 hours ago, captainbeefheart said:

 

15 watt 3k:8 output transformers are $30 each, $60

Power transformer is $48

The first couple of amps will come with GE 6550's as I already have them but two KT88's at $50 each is $100

 

We are at $208

 

That is if we go this route. I have  3k:8 Transcendar transformers here that I can breadboard with.

To clarify, are you going to use an ultralinear connection or pentode/tetrode screen connection for the SE amp?

[henry4841]

18 hours ago, captainbeefheart said:

Another option that I was doing for people for a little while was they purchase a Chinese tube amp kit, not assembled and have it shipped to me. I then wire up my own circuit for an additional $100. They end up with a really good sounding and performing tube amp for around $350. I know people think the transformers they send are garbage but they really are not. I got great bandwidth, low distortion and good stability quite easily with them.

 

https://www.ebay.com/itm/124915044864?hash=item1d15845200:g:u7YAAOSw-vlVi40P

 

 

I've assembled a couple of the Chinese amp kits and like yourself impressed with the iron supplied. The only problem I see with your pricing is shipping and tax which is going to add to the base cost of $195. 

[captainbeefheart]

6 hours ago, Curious_George said:

To clarify, are you going to use an ultralinear connection or pentode/tetrode screen connection for the SE amp?

 

If that's the direction this is going it will be a pentode/tetrode connection and not ultralinear.

 

5 hours ago, henry4841 said:

The only problem I see with your pricing is shipping and tax which is going to add to the base cost of $195. 

 

That's why I said if people wanted the 15 watt amp it would be closer to $400. The original goal was a $250 amp and probably around 5-8 watts, which could still happen but some have voiced they would pay more for the added headroom which will up the price. I don't blame them, I personally don't consider myself someone that listens to ear bleeding music but for the headroom for dynamic music I calculated out something around 6 watts for my personal listening requirements. I imagine I am on the lower end of things so I would rather have more headroom than less, 15 watts should be adequate for the majority of the forum members.

[grindstone]

The thing nobody talks much about is going after headroom in all the details.  I've not messed with amps for ages but I once looked hard at that stuff (as hard as a non-EE can, anyway).  I mean, headroom is always everybody's goal, but you have to pick some target numbers somewhere if you get serious.  Swing is one.  Energy storage & discharge rate is another (I see lots of people refillng their large caps with little caps).  People assume getting supply filter resonances/Q out of the passband and a flat-ish, low-ish Zout of the supply just handles things if they get close to continous conduction.  There's dragons in there with response and self-resonance of discrete stuff (or regulator "stuff").  Failure conditions/startup/one-tube-out blah blah.  Reflections of signal in the "line" (ie rail).  And somewhere you have to back-into it all from some target Death Transient (tm), itself based (for me anyway) on backward propagating the theater-people's specs at the listening position.  People do everything from mongo PT's and tiny filters that crazy-ring (but unload lickety-split while fostering a compander) to the opposite.  Yeah, all the normal "signal stuff" matters greatly, but the supply is still the amp.  You can slap regulators and independent supplies on each stage, but how many supplies do you want to build, etc.  So there's some assessments and tradeoffs in the things for a target cost spec, and it's those decisions wrt headroom that almost nobody either does or shares out-loud.  There are arguments to be made for every possible approach and there's no bottom to the topic when you include all the iron losses or trying to clean-up diode-switching or thinking about radiating/loop-area stuff.  Big-fun, if you ask me

[captainbeefheart]

23 minutes ago, grindstone said:

The thing nobody talks much about is going after headroom in all the details.  I've not messed with amps for ages but I once looked hard at that stuff (as hard as a non-EE can, anyway).  I mean, headroom is always everybody's goal, but you have to pick some target numbers somewhere if you get serious.  Swing is one.  Energy storage & discharge rate is another (I see lots of people refillng their large caps with little caps).  People assume getting supply filter resonances/Q out of the passband and a flat-ish, low-ish Zout of the supply just handles things if they get close to continous conduction.  There's dragons in there with response and self-resonance of discrete stuff (or regulator "stuff").  Failure conditions/startup/one-tube-out blah blah.  Reflections of signal in the "line" (ie rail).  And somewhere you have to back-into it all from some target Death Transient (tm), itself based (for me anyway) on backward propagating the theater-people's specs at the listening position.  People do everything from mongo PT's and tiny filters that crazy-ring (but unload lickety-split while fostering a compander) to the opposite.  Yeah, all the normal "signal stuff" matters greatly, but the supply is still the amp.  You can slap regulators and independent supplies on each stage, but how many supplies do you want to build, etc.  So there's some assessments and tradeoffs in the things for a target cost spec, and it's those decisions wrt headroom that almost nobody either does or shares out-loud.  There are arguments to be made for every possible approach and there's no bottom to the topic when you include all the iron losses or trying to clean-up diode-switching or thinking about radiating/loop-area stuff.  Big-fun, if you ask me

 

Yes lots to think about.

 

I usually make lists after I choose my topology. Let's face it a single ended class A amplifier is going to have much different design goals compared to a Class AB push pull amplifier.

 

People usually start out building single ended amps because they are 'easier' but that can be a myth, yes there are less parts count for the most part but you don't have the luxury of output stage PSRR like you get with push pull. The signal passes directly through the power supply for single ended. Yes if you use chokes in the power supply make sure the resonance is below the audio frequency, it should be below 10Hz. Using a tube rectifier usually means smaller filter caps so make sure your choke is large enough to keep the resonance <10Hz. Silicon diodes for rectifier? ok fine just snub the rectifier induced ringing with an RC filter across the secondary winding. Class A single ended will need a power supply with a low enough impedance to pass all frequencies through it and back to the cathode. This is often a miss with many builders using small filter/decouple caps. Class A will not require regulation like a class AB amp.

 

From my experience with low powered single ended class A amplifiers one major problem is the the output stage going into grid conduction and creating a bias shift and blocking distortion. Many just do not put forth the extra effort to DC couple the output stage with a low impedance driver that can source current for the grid. I spent yesterday looking online at single ended amplifiers on the market and almost had a heart attack at the prices for performance. I won't name names but many do not tell you any distortion figures. I was seeing 8-10 watt SE amplifiers with no feedback (lots of distortion) and class A1, no grid current drive for the output stage going for $6,000. Having a look inside it has a tiny printed circuit board with the 6SL7 and KT88's, it's the simplest amplifier design you could possibly imagine with poor performance and costs $6,000!!!! Really?? C'mon there is no way that amplifier is remotely even close to costing them maybe $500 to make. Low and behold I saw 'Jensen' coupling capacitor going from the 6SL7 stage to the KT88, I guess that is their idea of performance improvements. I am not saying you need to have a ton of complicated circuitry but with the price tags for these amplifiers I would expect at least an interstage transformer since they aren't using feedback anyway. Just that alone would have helped performance substantially. It's cathode bias which is fine, I have no real issues with cathode bias but for that price range I would expect fixed bias with a nice meter and control to adjust bias as for anyone that knows anything not every tube will bias up perfectly with cathode bias, so people are going to be either too cold or too hot more often than perfect. I would prefer perfect if I have a $6,000 amplifier. I mean really a two stage no feedback AC coupled extremely dead simple as you can get amplifier with awful performance demanding huge money? The owner at least admits he is a social worker and not an engineer, yes we can clearly see that from the designs. I found a handful of these companies out there selling extremely expensive single ended tube amps with awful performance, but hey they sure do look the part. I will admit they did look very nice but what's the point of lipstick on a pig?

 

This only pushes me even more to get this project rolling forward, to get in the hands of people something that will sound MUCH better for 1/10 the price tag. I truly think these people are marketing whizzes, look the part and as a big price tag because people clearly feel the more they pay the more they get. It's looking like that isn't the case in tube audio world lately.