"El Cheapo Valve Phono" using 6ER5=>12AT7WC? - diyAudio
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Old 25th September 2005, 04:41 PM   #1
fred76 is offline fred76  Philippines
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Default "El Cheapo Valve Phono" using 6ER5=>12AT7WC?

Hello,

I'd like to do a similar project to KYW's "El Cheapo" phonostage:

http://www.diyaudio.com/forums/attac...amp=1077837811
http://www.diyaudio.com/forums/showt...624#post335624

The only thing different Is I'd like to use 6ER5 for the 1st stage and 12ATWC for output. I have these tubes on hand. This would be similar to K. Strain and G. Pimm's phonostage with 6GK5 2nd stage GL biasing. I'd like to try it without the Active-Loading though. KYW had design input years ago on Kurt Strain's phonostage, hence the El Cheapo came into fruition...

Anways, this would entail changes in values not just for the tubes but for the Riaa filter itself. I still have to read up on Kurt's tutorials here:

http://www.sonic.net/~ktstrain/RIAA_eq.htm
http://www.sonic.net/~ktstrain/Groundtutor.htm

..and KAB's filter calc. to help out:

http://www.kabusa.com/RIAA.HTM

With a help of a buddy, hopefully we could come up with something. I currently have an old model Wright phonostage that uses the same 6ER5's, it sounds best with the 12AT7WC as mu-follower output compared to the other tube types "rollable' with it. I would sell the Wright soon, as the circuit is too colored for me now having more revealing amp-speakers these days etc.

What do you guys think of the 6ER5 and 12AT7WC combo using the El Cheapo style circuit from a teccnical standpoint??? Do you think an all 6ER5 two stage circuit is a safer bet?

Thanks for any feedback or advice....

fred
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Old 27th September 2005, 01:06 AM   #2
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There's some design info on my page here:


6C45P Phono Page

-j
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Old 27th September 2005, 04:20 PM   #3
fred76 is offline fred76  Philippines
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Hi Jeremy,

Thanks for linking your webpage on phono designs - cool stuff... I see that you have a 6GK5 design, but I have decided not to use batteries for phono this time.


I found a full 26DHT preamp schema below using the active-loaded phono desgn, BUT it also indicates "R1: 4800" at the plates. I assume they are the measured plate resistance with the CCS loading. The question is If anyone has a schema that uses NO CCS. I have nothing against CCS it's just that I want to start with the basics first.

Anyways, I hope somone has the Non-Ccs version of the basic phono design below .. Or at least have suggestions for a 6ER5=> 12AT7WC variant, I mean like values for the last Decoupling RC filters and plate supply V after the +250V like in KYW's schema, and Output Cap and R value, etc??? Thanks!

http://www.diyaudio.com/forums/attac...amp=1077837811

Click the image to open in full size.
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Old 27th September 2005, 05:33 PM   #4
EC8010 is offline EC8010  United Kingdom
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There won't be such a thing as a non-CCS version of Gary Pimm's design. The CCSs allow high gain from only two valves, low distortion even though a low HT voltage is used, low (and unchanging) output resistance to the RIAA network and following stage. Fundamentally, it's those CCSs that allow the design to work - use the design as the designer intended...
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Old 27th September 2005, 06:25 PM   #5
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by fred76
The question is If anyone has a schema that uses NO CCS. I have nothing against CCS it's just that I want to start with the basics first.
Changing this circuit to a non CCS Loaded design is rather trivial.

Load the first valve with 39K Anode Load and the second with 56K.

Then change the RIAA EQ Circuit to ten times the resistor values and one tenth the capacitor values, without the "Mu-Follower" output of the CCS you cannot drive the low impedance RIAA Network.

Finally, the first series resistor needs some tuning to account for the effective anode impedance of the input valve, it looks like this is around 15KOhm (including the effect of the unbypassed cathode resistor), so that needs subtracting from the series resistor, final tuning needs to be done with measurements taken.

Doing as instructed will result in around 2 - 3db less gain and obviously the output impedance of the Phonostage is up to also around 15k, a bit high for my taste, at the levels most phonostages operate at linearity is a non-issue.

Using the ECC81 in the second stage is not that good an idea either, it too has high output impedance.

The choices I made for "Valve El Cheapo" with ECC83 in the input and ECC88 in the second stage make a good deal of sense.

You could adjust the first stage to use 6ER5 or 6GK5, but be aware that the anode impedance of the ECC83 plus it's cathode resistor form part of the RIAA EQ, so you would have to increase the series resistor in the RIAA EQ.

Sayonara
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Old 27th September 2005, 09:44 PM   #6
fred76 is offline fred76  Philippines
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Thanks for the comments/education guys…


Quote:
Using the ECC81 in the second stage is not that good an idea either, it too has high output impedance.

Hi KYW,

Just scrapped the 12AT7WC idea..


Quote:
The choices I made for "Valve El Cheapo" with ECC83 in the input and ECC88 in the second stage make a good deal of sense.

Yes, that’s why that’s the first I'd like to try out…


Quote:
You could adjust the first stage to use 6ER5 or 6GK5, but be aware that the anode impedance of the ECC83 plus it's cathode resistor form part of the RIAA EQ, so you would have to increase the series resistor in the RIAA EQ.

At least technically this is one is doable… I would try a 6ER5 input with the 180k/10uF=>39k anode-load R and a GL biased 6922 2nd stage. Apart from increasing the first series R (the 215k in 12AX7's case??) would there be other change/s in the RIAA filter or only the values itself? What are the other crucial parts in the circuit must I pay attention to for this 'version'? Like should I still use grid-stoppers for the 6ER5's (at least 100R)? Thanks again…

Regards,
fred
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Old 27th September 2005, 10:50 PM   #7
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Hello,

there is a very helpful little RIAA calculator available free at www.beigebag.com.

This is originally from Tubecad and it calculates passive combined and split RIAA stages and even accounts for load and source resistance. I have found it to be very accurate in corresponding with the results of a respective spice simulation.

Oh, how I wish I had a working phono stage! My everlasting quest to remove all capacitors from the signal path has had me tinkering for two years now on a phono preamp which goes something like this:

EL84 ->(DC coupled)-> EL84 -> 5:1 transformer (Sowter 9040) -> Tango network (self made) -> 4:1 step-up transformer (Sowter 4383 - very good btw.) -> EL84 -> 5:1 transformer (Sowter 9040)

While it does sound good it is bitten by some stability/noise/microphonics issues. Besides it is mono...

So I am also looking for simple phono stage in order to at least listen to some records while I optimize the design. (And I can't stand the sound of the active transistor eq in my NAD 3020 - in fact I can't stand the sound of the 3020 at all - this heavy negative feedback hurts my ears after a while)

I was thinking along the lines of the original RCA circuit but it has a prohibitive output impedance of about 30K so it needs a buffer stage and a cathode follower is not compatible with my philosophy of sound. Besides I would like to reduce the size of the input resistor of the RIAA network because I feel that a lower value here would be beneficial. This is why I designed a stage around the ECC81/12AT7 like this:

Click the image to open in full size.

It sports an output impedance of about 8K which should be enough to drive a 50K load and the network input resistor is only 100K as opposed to 470K on the RCA circuit. Its gain should be better than 40dB (100x). Now the ECC81/12AT7 is not my favourite tube but I was looking for a compromise to the shout of those high mu tubes. I really wonder how it would sound?

Meanwhile after discovering KYW's circuit, which I find very intriguing indeed, I am not so sure which track to pursue for a quick and simple RIAA? Will the ECC88 make up for the sound of the ECC83? Or is a 3 stage design the way to go? What do the experts say?

Best regards
Michael
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Old 27th September 2005, 11:10 PM   #8
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Fred76:

Glad you enjoyed your web visit. I have another pretty simple, but still DC-coupled design up in a different place (I've decided to colonize all of cyberspace bit by bit) at my weblog:

The Tubegeek

Before the gang starts poking holes in the design, let me make a couple of points:

1. In my system, it's only driving a couple of inches of twisted pair into a 100k volume pot, so the highish output impedance is something I'm OK with,

2. There is a battery (in series with the signal this time),

3. The whole thing (especially point #2) is a ripoff of John Atwood's very well-regarded Artemis Labs PH-1 phono stage. I wanted to see whether I could get good sound using some of his ideas while keeping things very simple.

It uses 6N1P into 6SL7 and sounds quite good to my ears. It's certainly simple enough to recommend as a project for someone looking for a quick & dirty phono.

The 6N1P uses twice the heater current of 6SL7, 600mA vs. 300mA. I have a DC supply feeding the heaters and right now I'm burning 300 mA up in a resistor paralleling the 6SL7 heater. I'm thinking about paralleling two 6SL7's instead to halve the output impedance, I just haven't had time yet to cut the hole for the additional octal socket.

Comments, hole-poking, etc., welcomed.

-j
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Old 28th September 2005, 03:06 AM   #9
fred76 is offline fred76  Philippines
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Hi Michael,

Thanks for the link! I don't know if you've seen Steve Bench's no C phonostage using Lundahls... You might want to check it out. I also remember the olde (r.i.p.) webpage of Mick Maloney (now Supratek of Oz) were he had design exchanges with KYW; among them I THINK are LCR type and no C phonostage/s. Thomas Mayer also had some ideas/drawings (one using DHT's in a phono!) I've seen before, I just can't remember if they are no C types. It sure has a lot of Lundahl iron ($) in it though. He must have a major discount from them by now.

Thanks for sharing your simple circuit, it just gave me an idea to try ECC81's at the input =). Maybe you could try grafting in an ECC88 output stage there just to try it out. Circa 38-45dB is actually enough for my needs. I use step-up trannies for MC.


Hi J,


Nifty blogsite =) Anyways, you guys are prolific in designing/building, the only thing I ever did was build a half-kit and tweak/modify from others suggestions. So I'm not even qualified to poke a single hole in your Atwood inspired design...

I actually did implement grid batt bias in the input of my amp but directly bypassed the coin batt w/ a small cap + grid R (not to ground). It works well w/ line level input signals but I'm not sure if the 100k series R attenuator (volume control) is a "good thing" for it as opposed to just using a fixed input R.

I'm not a real fan of Supergrass, but OTOH I just pulled out my olde "Road to Ruin" lp. It would be Joey Ramone's death anniv in a few days I think...
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Old 29th September 2005, 01:03 PM   #10
fred76 is offline fred76  Philippines
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I've come up with a reg HT supply for either of the mentioned valves above which I could step down for proper heater V's (w/ mild underheating). The second one is just a variation of the first.

I could use LM or LT1085. Any comment/s appreciated.

Click the image to open in full size.

Click the image to open in full size.
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