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Author Topic: Using old tube preamps with "Tape Head" inputs  (Read 16030 times)

Offline stellavox

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Using old tube preamps with "Tape Head" inputs
« on: February 02, 2007, 07:46:53 AM »
Most old (sorry; Vintage)  commercially produced tube preamps had switchable low level inputs to accept both magnetic phono cartridges AND tape heads.  Some, like the Mac's and Marantz had a control on the back which varied the high frequency equalization and could now be adjusted for either NAB or a close approximation of IEC.  They all had those "dreaded" tone controls and/or switchable high and low frequency "turnover" points which enable even more flexibility. 

Anyone tried "interfacing" any of these with a "modern" transport?  Only problem I've run into is low output; reproduce heads of that era had higher inductance and higher output versus the lower impedance, lower output heads designed to be used with transistors.

Let us know!!

Charles     

Offline James Guillebeau

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #1 on: February 02, 2007, 11:38:29 AM »
Most vintage were in feedback loops around 12ax7A circa Ampex, Magnecord, Scully, Presto, Stancil Hoffman, RCA. You need at least a 400mh playback head for good output into a direct coupled tube stage, preferably 650 mh but not higher. This puts the head resonance within the audio pass band and results in lowest noise. The other way is the use an input transformer with a very low inductance head; 4.5mh. This also yields good SN ratio. With the advent of multi track heads the closeness of the head stack put a limitation on the amount of wire they could cram into a head coil.  It's difficult to make a 650 mh multi track head. The Studer C37 used mostly passive EQ with a minor feedback correction. If you had an NAB or IEC EQ test tape then you could make several of the resistors variable and then set the proper EQ. You also need to check the time constants and maybe replace a few caps.  Cheers! James
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Offline classicrecordings

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #2 on: March 09, 2007, 03:31:26 PM »
I re-wired a stock 4 track Revox A-77 MKIII directly from the tape heads to the output jacks and then ran 2 foot leads into my Harmon Kardon Citation 1 pre-amp's tape head input.

The resulting sound was a revelation compared to any 4 track machine I have ever heard on any system. I double checked this by playing back my 2 track tapes on the 4 track machine, and then I compared the sound of those recordings by playing them back on my 2 track Revox with it's internal pre-amp going into the standard tape input on the HK, as well as directly into my power amps, bypassing a linestage pre-amp altogether. Without an exception, the 2 track tapes always sounded better played back on the 4 track machine going through the tape head input.

Of course that is just my experience.

David
David Stangret
HK Citation 1 Pre-amp, Thorens TD-124, Ortofon 309S arm, SPU cartridges, mono block tube amps, Ars Acoustica LaDiva speakers, and I'm still trying to decide what machine to go with.
http://www.classicrecordings.ca

Offline ironbut

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #3 on: March 09, 2007, 08:56:01 PM »
Hey David, welcome. It sounds like you've come to the right place! Do you know the output of the A77 playback heads? I used to have a Mk2 that I had converted to 1/2 track. I was never that crazy about the sound of the playback amps but I loved it just the same.
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
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Offline High and Outside

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #4 on: March 11, 2007, 07:19:07 PM »
I re-wired a stock 4 track Revox A-77 MKIII directly from the tape heads to the output jacks and then ran 2 foot leads into my Harmon Kardon Citation 1 pre-amp's tape head input.

The resulting sound was a revelation compared to any 4 track machine I have ever heard on any system...

David

Ooh, now you've got me thinking. I also have a Harman-Kardon Citation 1. Hadn't thought about connecting a deck right to it and using the internal preamp. I'mana have to try that.

(and how's this for irony: I acquired the Citation I in a swap with Stan Ricker...for some tape electronics he wanted.)
Paul Stubblebine
Managing Director, The TapeProject

Offline James Guillebeau

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Citation 1 preamp has enough parts to make 3 preamps
« Reply #5 on: March 12, 2007, 09:24:01 AM »
The front end is two 12ax7s with both feedback and passive EQ between stages. I think much better pre-amplification could be found. It was great when good 12ax7s in the late 50s could be had. I would favor a much high mu front end as a equivalent input noise consideration.
"I'm completely free of prejudice. I hate everyone equally." W.C. Fields

Offline PJ

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #6 on: March 12, 2007, 05:42:23 PM »
Just a detail - it's higher transconductance (gm) that gives lower noise. Mu is gain, which is usually not a problem as long as it's 20 or more.

Tube rush is not that severe a problem though. We're using an EF86 in our head amp, which is relatively low transconductance and a pentode to boot, and with the lower-inductance heads. Tape self-noise is still significantly louder than the preamp's intrinsic noise.

Of course many tubes have too much excess noise (1/f noise) due I believe to cathode impurities, but tested quiet tubes are still available for a modest premium.
Paul Joppa
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Offline James Guillebeau

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The EF86/6267 is an "old" solution. But then again most everything surrounding tubes is "old". I can't help but think if you are sitting on such excellent RIAA preamps, the EIA and NAB curves aren't that far off. By the way what in your experience is the lowest grid leak champion among hi gm tubes? Thanks James Guillebeau
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Offline stellavox

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James what's wrong with "old" solutions
« Reply #8 on: March 13, 2007, 03:54:47 PM »
What "new" solutions do you propose???

Charles

Offline James Guillebeau

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #9 on: March 13, 2007, 04:52:25 PM »
I'm toying with an all FET cascode front end which offers lower noise using n and p channel jfets from Toshiba.  Thanks James
"I'm completely free of prejudice. I hate everyone equally." W.C. Fields

Offline classicrecordings

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #10 on: April 12, 2007, 12:21:51 PM »
Hey David, welcome. It sounds like you've come to the right place! Do you know the output of the A77 playback heads? I used to have a Mk2 that I had converted to 1/2 track. I was never that crazy about the sound of the playback amps but I loved it just the same.

I can't say for sure, but I think the playback head measured at 1.5 mV. I don't have that 4 track anymore, so I cannot check for you.

David
David Stangret
HK Citation 1 Pre-amp, Thorens TD-124, Ortofon 309S arm, SPU cartridges, mono block tube amps, Ars Acoustica LaDiva speakers, and I'm still trying to decide what machine to go with.
http://www.classicrecordings.ca

Offline classicrecordings

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Re: Citation 1 preamp has enough parts to make 3 preamps
« Reply #11 on: April 14, 2007, 11:18:09 AM »
The front end is two 12ax7s with both feedback and passive EQ between stages. I think much better pre-amplification could be found. It was great when good 12ax7s in the late 50s could be had. I would favor a much high mu front end as a equivalent input noise consideration.

Actually, the Citation1 has two phono inputs, each with their own pair of 12AX7's. I have not found the tape head input too noisy. I think the key is to have premium tubes in the phono stage which is where the tape head signal goes through.
 
My Citation1 has been completely re-built by a friend using Jim McShanes updates, as well as removing some other things from the signal path to clean up the sound. Though I do not believe that it is as clean as a dedicated BottleHead, it is better sounding than the internal electronics of any machine I have heard so far.

David
David Stangret
HK Citation 1 Pre-amp, Thorens TD-124, Ortofon 309S arm, SPU cartridges, mono block tube amps, Ars Acoustica LaDiva speakers, and I'm still trying to decide what machine to go with.
http://www.classicrecordings.ca

Offline ironbut

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #12 on: April 14, 2007, 09:19:44 PM »
Yeah, Jim is a great guy and I love sending business his way. Does the Citation have enough gain? I'd think the 12ax7's should. I've tried a lot of 12ax7's and my fav's are either Mazda grey plates or orange globe Holland, Amperex. People always stick their noses up when I mention the orange globes but I guess it's personal taste. Their real punchy and make it easy to tell what kind of guitar ( I used to play classical ) a players using. BTW, I have some 1956 Bugle Boy d getters but I which are very nice and maybe more neutral,and extended  but I love those orange globes! If you can get away with it, Sylvania and Raytheon 5751's are nice too but sometime their lower gain will throw off the passive eq in a phono/tape.
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
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Offline beefman

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #13 on: April 19, 2007, 07:21:01 PM »
The Manley tapehead is all built on 12AX7s is it not?
Jeff Kane
Owner of many decks; in possession of few!

Offline xcortes

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Re: Using old tube preamps with "Tape Head" inputs
« Reply #14 on: August 31, 2009, 05:07:57 PM »
I got my hands on a Fisher 400 integrated (together with a pair of Altec two way monitors from the late 60s  - don't ask -). Since my Technics machine is not working yet (Although Steve and I are getting there) I tried my Tascam via SUTs. I haven't done any serious listening tests but I've played a couple of prerecorded 2 track tapes through this "system" and so far it seems to be a keeper for the bedroom while I finish some other projects that at current pace could take years.

And in line with previous posts of the discussion the Fisher 400 pre is also built around 12aX7s.
Xavier Cortes