Cheap Forum Amp by Captainbeefheart

[henry4841]

Just an example guys. About 20 years ago me and a friend of mine started a collection of old military rifles being dumped on the market at really cheap prices. There were an abundant supply of them for 70 to 140 bucks each at that time. The one I paid $140 for now is selling for 7 or 8 hundred dollars. Just think of the money I could have made if I had bought a 100 of them. Old tubes one day will do much the same thing. A wise young man will stock up on those old audio tubes that you can buy cheap now. Probably as good an investment you will find. Nelson Pass buys up almost all the good old transistors made in the 70's he can find to build an amplifier from one day. Especially those old static induction mosfets. Tube guys should probably think the same way.  

[captainbeefheart]

6 hours ago, henry4841 said:

Guys I have told ya'll about a really good sounding tube, the 6Y6, that you can buy cheap for an old tube made long ago and made specifically for audio. The general consensus from all that I have read is those old tubes sound better than the new production tubes you can buy. Most say if you install a set of new old tubes in any amplifier you own you are going to experience another level with your amplifier. I hope one of you will listen to an old man and design your circuit with your twist using this 6Y6 tube. Call it the Super Sweetie or Hot Rod Sweetie. I think you get the idea. The price of it is only going to go up never down and right now they are readily available at a cheap price. With the Deckert amp being so much in demand a little amp designed around an good old audio tube should be very desirable. 

 

This is about the cheapest price you will find for a NOS tube for Deckerts amplifier, $80. Let me see, 80 bucks or $10 for an old tube to build upon. 

 

https://www.tubedepot.com/products/6bq5-el84-usa

 

 

I have a quite the collection of tubes and was always purchasing NOS tubes especially from the what was called the "dollar days of summer" or something from ERSC where they put out a list of tube for $1 each. I stocked up on TONS of fantastic tubes that aren't that well known to the audio community that work well for audio.

 

I have lots of 6Y6 tubes but some reasons I choose other tubes I have to build with are; 1) Maximum plate voltage for the 6Y6 is 200v. 2) Maximum plate dissipation is 12 watts. Transconductance is slightly higher than a 6L6  and plate impedance in pentode mode is 18k which isn't that bad. It's a good tube don't get me wrong but I have found lots of sleeper tubes that cheaper to buy and outperform it. One such tube that is really a sleeper is 6AV5GA, I know Maynard says that the 6Y6 triode strapped is similar to a 2A3 but the closest I have found hands down is the 6AV5GA, it's literally exactly like a 2A3 so much so that when you overlay the plate curves they look identical on a curve tracer. The 6AV5GA can handle 500v on the plate and withstand 5500v pulses on the plate!! It's a sweep tube so ya it's built rugged, the plate can withstand quite the abuse, datasheet says 11 watts but that's for horizontal/vertical deflection amp which are abusive, for audio it's more of a 22-25 watt dissipation tube. If you compare the pentode curves between the two the 6Y6 looks kinda wonky but the 6AV5GA look clean and flat, almost like a Mosfet which is impressive for a tube.

 

I'm not putting it down, it is a nice sounding tube in triode mode for low power but that's really the problem is 1-2 watt limited. There are A LOT of great tubes out there cheap that aren't really talked about outside of certain circles. I completely recommend you keep on experimenting with different tubes other than the regular audio tubes you typically see. Tube technology peaked in the Television era and so most of those tubes are quite amazing really, audio was considered mundane with relaxed requirements so "audio" tubes are really nothing special. The TV tubes were very expensive in that era but because there are no TV's with tubes anymore and there are warehouses full of these tubes the price dropped to peanuts just to get rid of them but they can certainly be used for audio with excellent results bettering audio tubes.

 

I won't list too many out in the open so I can give you more names of tubes to try in private message if you like.

 

 

[Khornukopia]

31 minutes ago, captainbeefheart said:

 

This is my thread ...

 

Yes, but it is on the Klipsch.com forum, so all of us here needs to follow their guidelines.

[captainbeefheart]

@henry4841

 

Back to tube talk.

 

Here is a little back story to how we got turned onto the triode wiring the 6AV5GA in replacement for a 2A3.

 

So in the early days we had the 2A3 and the 6 volt filament version of the 2A3 which is 6A3. Same plate curves just different filament requirements.

 

Then came along the octal version of the 6A3 known as the 6B4. 

 

At some point some of us were finding 6AV5GA's that came internally wired as a triode. Someone, I believe it was George Anderson got some information that since RCA was already in production of 6AV5GA they just triode wired it internally and sent them off as substitutes for 6B4G's. So even RCA knew that electrically that tube when triode wired performs just like a 6B4 which has it's heritage linked directly back to a 2A3.

 

 I thought it was pretty interesting. This also gave me the idea a while back to make the Brook 12a clone with that tube instead of the expensive 2A3.

 

Fun with tubes

[Curious_George]

On 8/16/2022 at 1:37 PM, captainbeefheart said:

 

Yes Nick sent me the details the other day, they will be added to the list of possible transformer vendors for the build. One concern I have with them is they are guitar amp transformers rated down to 40Hz. I'm assuming this is for the rated 10 watts so myself or I believe Nick said he got 4 of them so maybe he can do some testing to see distortion vs output power at 20Hz with different DC bias currents. Another concern I have is the 50mA rating, for a 10 watt SE hifi output transformer I'd like to see at least 80mA or 100mA even better. I'm sure we can work around these concerns and still achieve good results but they designed those transformers around the guitar amp market. For the 6550 circuit we need a lower load impedance like 3k and it needs a minimum bias current of 80mA and preferably 100mA. We just need to pick a different power tube. Nick was thinking 6V6 which should work better with them, bias current of 6V6 wouldn't be more than the 50mA. BUT, and there is always a but, 20H inductance isn't really great for such high impedance tube like the 6V6 which is 50,000 ohms or higher, sometimes as high as 80,000 depending on operating point. At 20Hz the impedance of 20H is down to half the reflected (and optimal) load impedance of 5k at 2.5k. Basically the low inductance starts to dominate at low frequencies as the load impedance (2.5k) while mid band frequencies see 5k. It would be nice to see 40H of primary inductance for a 6V6 build. It's tricky because most tubes that want a 5k load will have high plate impedance and so need more than 20H, more like 40H. They are guitar amp OPT so I know they probably designed them this way on purpose. Maybe he can wind us a 2.5k load impedance with 20H that will work with an EL34 but we would need him to probably up the bias current a tad to 80mA. Even if they were $50 each that's still a great price. One of us should open up a correspondence with him and see if he would wind us something to our spec and price it out for us.

 

There are other options like parafeed. I have some 10 watt constant voltage PA transformers that are cheap as dirt. I was getting good results with them, easily 5 watts output with a gyrator load and AC coupled. I don't think anyone will complain about spending $6 on an output transformer. I'll see if I can get the full 10 watts out of one at 20Hz but I doubt it.

 

 

Matt at Musical Power Supplies indicated he could wind to a specification. I have some correspondence with him, so I can contact him again. Give me the spec Cappy and I'll send it over to him. 

[captainbeefheart]

28 minutes ago, Curious_George said:

Matt at Musical Power Supplies indicated he could wind to a specification. I have some correspondence with him, so I can contact him again. Give me the spec Cappy and I'll send it over to him. 

 

Wow! Great news Nick and thanks for asking him.

 

Here are the specs:

 

Load: 3k:8

 

DC current: 100mA

 

Inductance and power: Flat to lets say 35Hz with a source impedance (Tube plate impedance) of 27k and  15 watts into 8 ohm load.

 

Depending how he winds it and how much copper vs core he uses is what will set the high frequency response. The circuit employs 20db of feedback so bandwidth is great even if open loop bandwidth isn't the best but of course we want low leakage inductance and low interwinding capacitance.

 

 

[Curious_George]

4 minutes ago, captainbeefheart said:

 

Wow! Great news Nick and thanks for asking him.

 

Here are the specs:

 

Load: 3k:8

 

DC current: 100mA

 

Inductance and power: Flat to lets say 35Hz with a source impedance (Tube plate impedance) of 27k and  15 watts into 8 ohm load.

 

Depending how he winds it and how much copper vs core he uses is what will set the high frequency response. The circuit employs 20db of feedback so bandwidth is great even if open loop bandwidth isn't the best but of course we want low leakage inductance and low interwinding capacitance.

 

 

I sent the spec to Matt.

[captainbeefheart]

3 minutes ago, Curious_George said:

I sent the spec to Matt.

 

Thank you!!  Let me know if he needs any more information.

[jjptkd]

On 8/18/2022 at 12:24 PM, captainbeefheart said:

I'm back, life threw me in the spin cycle for a bit but I'm getting caught up.

 

Here is the thread to Shakey's amp:

 I

 

Once I ship it I'll start a thread on Chris' amp.

 

Thanks guys for worrying about me.

 

 Another month has gone by (9 total?) and all we have here is a handful of members with empty pockets. From what I understood from Shakey's amp thread from 2 months ago the chassis was painted, amp circuits were done it just needed reassembly then test and ship, not sure what has happened? Looks like the Capt hasn't logged in for almost 3 weeks, not looking good. 

[OO1]

YEP 

[captainbeefheart]

Hey guys I'm okay now.

 

I awoke early one morning in severe pain again and shortness of breath, turned out the hole didn't fix itself and my lung collapsed again worse this time. I went through a procedure where they glued the hole shut so hopefully it won't reoccur ever again. They did originally tell me I had a 1/4 chance of coming back which it did, not fun let me tell you.

 

I charged my phone yesterday and turned it on, I didn't see any texts from Shakey or Chris if they were trying to get a hold of me.

 

I'm also under moderated posts so I don't know when anyone will see this but I figured I would check in here anyway and see what's going on.

[Schu]

16 hours ago, captainbeefheart said:

Hey guys I'm okay now.

 

I'm also under moderated posts so I don't know when anyone will see this but I figured I would check in here anyway and see what's going on.

 

we all can clearly see what is happening...

[Marvel]

1 hour ago, Schu said:

we all can clearly see what is happening...

not really but that's ok

[babadono]

"Its all wrong but its allright"  ?

[hanksjim1]

To be fair and update the record, @captainbeefheart did refund my money yesterday for a moving coil step up transformer deal that didn't work out. 

 

I hope the amp guys also get some satisfactory resolution.

[Travis In Austin]

Does this cover all of this now? Projects and/or transactions by @captainbeehheart?

 

One amp to go is that correct?

[Shakeydeal]

36 minutes ago, Travis In Austin said:

Does this cover all of this now? Projects and/or transactions by @captainbeehheart?

 

One amp to go is that correct?

 

Two amps. Mine and @Westcoastdrums

 

I understand from the captain we are getting close to wrapping things up. I hope there will be good news to post in the near future.

 

 

[jjptkd]

25 minutes ago, Shakeydeal said:

I understand from the captain we are getting close to wrapping things up. I hope there will be good news to post in the near future.

 

 Glad to hear this, really hope it works out. 

[Travis In Austin]

11 minutes ago, jjptkd said:

 

 Glad to hear this, really hope it works out. 

Me too.

[captainbeefheart]

4 hours ago, Travis In Austin said:

Does this cover all of this now? Projects and/or transactions by @captainbeehheart?

 

One amp to go is that correct?

 

Since both Shakey (Tim) and Westcoastdrums (Chris) asked me to make custom amps the same day I felt it only fair to ship both amps at roughly the same time. As of right now Chris' amp is a little behind Tim's so his will go out 2 weeks after Tim's. I didn't want to show favoritism to one person or the other by building one amp before the other so I'm doing both at the same time.

 

It's a lot of work but I like doing it this way because when I was stuck on one amp, say deciding which tube to use based solely on sonics and not simulation or calculated data I would bring the breadboard to  my listening room and listen to it while I switched over to work on the other amp. Basically I could listen to one circuit while working on the other.

 

I don't even start building the actual amplifier until I have the circuit design 100% completed. The reason is because how can I optimize the layout if I don't even know what the circuit will be? Many guys will have a schematic picked out that they found and will build it, then kinda tweak things inside the built amp until they are happy and then it's done. I don't like doing it that way because often times if you try and apply an afterthought you are limited on layout choices since everything is already built in the amp and holes are punched etc......

 

Tim's amp had the most design time  put into it for a couple reasons. The point of a custom amplifier is to make it work best in the application, we know the load is La Scala's but the source being either passive or active would ultimately have a huge impact on the amplifier design. With an active preamp I would be able to decrease the sensitivity of the amp quite a bit since there were gobs of signal swing available at the output to use. That and the fact it has a very low output impedance I could reduce the input impedance of the power amp to reduce noise floor even further. We went from Passive design to active when Tim got his active Benchmark preamp but it was after a few weeks or so that Tim realized his ears did not like the Benchmark and so he had a custom passive preamp made. I needed to go back to the drawing board to increase the input impedance of the amp and also increase the open loop gain. I was breadboarding lots of different circuits to get enough gain, from two cascading gain stages which I didn't really want because an extra gain stage will increase phase shift so keeping the voltage amp to just one stage was ideal. I tried different sharp cutoff pentodes, I tried two triodes wired as cascode (which behaves like a pentode but with better noise figures). My ear settled on the 6SJ7 pentode.

 

That wasn't the hard part either. The tough part is Tim's amp is single ended Class A2. What the heck is A2 you say? It means the power output tube is not AC coupled and the driver circuit can drive the it into positive grid territory where the impedance drops drastically and the grid starts to draw a lot of current. Class A2 is a huge advantage with lower powered amplifiers in that if you do end up running the amp near normal clipping would occur (G1=0v) it won't produce the nasty blocking distortion and bias shift caused by normal A1 operation. Plus you get more output power as a bonus. The crummy part about going this route is just about all the electrical models of tubes out there for simulations do not simulate grid current territory so I needed to do ALL the research on the A2 driver the old fashioned way, on paper and then breadboarding, bench testing and finally listening tests. It takes up a lot of time.

 

What I was trying to sort of compete against was a popular amp called "Bigger Ben" by ampsandsound. That amp with KT88/6550 produces 8 watts with 5% THD and costs nearly $6,000. My amp with the same power tube will do 12 watts with 1% THD and go up to 20 watts when nearing 5% THD, but only costs $1,200. Little bit better specs I think This wasn't the budget amp, Shakey wanted the very best parts and a nice exotic wood chassis with custom BMW paint. The output transformers were a huge chunk of the cost.