Bottlehead Tube Repro - how does one adjust the HF response?

[stellavox]

OK I found the information on the preamp on this website.

Now I've hooked up a Stellavox with a Pro Nortronics 2 track head wired out to it.  HF response is down 2 dB @ 10Khz and 5 dB @ 20Khz on both channels and this happens with the switch in either the NAB or IEC position (using the correct EQ test tape).  Took off the top and can't find any internal adjustments.  Would you please tell me what component(s) to "tweak" so I can dial in the HF properly.

Thank You

Charles   

[stellavox]

Sorry,

Forgot to mention - it's a new head and the same "front end" measures ruler flat to 20Khz on a Cello preamp (both NAB and IEC).

Charles 

[docb]

The standard Tube Repro is tuned to work with stock Technics and Otari heads and Flux Magnetics extended response heads. Different heads and cables can have different the response. You can try putting a 250K trimpot in place of each 47K resistor at the inputs. We have done a few this way where heads or head/cable combinations were being used that needed a different load to get the top end flat. Older Ampex heads are a little rolled off on top with a Repro or a Seduction. On the other hand my Nagra T is a bit hot at 15kHz into a Repro. IIRC Paul's A80 heads needed some trim too, though I can't remember how we ended up loading it.

[jcmusic]

Hey Doc,
I am expirenceing a similar situation with my seduction, it seems that when the switch is in the NAB position the highs are extreme. Almost unlistenable, then in the IEC position with a TP tape better, but I am not sure if it is correct. Have any suggestions?

Jay

[stellavox]

Doc,

I don't know if playing with the input R (47K seems awfully low) will fix this problem.  Not wanting to have to spend hours "reverse engineering" - does this design use passive EQ like the Seduction?  If so isn't the HF time constant that "R2-C1" mentioned in the mod article? so "tweaking" R2 should affect HF response? -  and what what/where are these components. 

I guess I have to ask why some "adjustability" wasn't built into this ($4000) unit?  What is one supposed to do as heads wear?

Thank You

Charles     

[stellavox]

As I don't have a lot of time to spend on this (the owner let me borrow it so a few of us could have listening session tomorrow between this and a few other head / preamp combinations), I did some quick "tweaking".  As I figured, changing the input resistor from 100k (in this unit) to 220K did nothing for the HF (and did increase the LF head bump) so I left the 100K in place.  Appears that the components affecting the HF are mounted on a little turret board on the front panel near the EQ switch.  The HF "pole" components are a 47 NF cap in series with a 1.06K resistor, which gets shunted by a 2.48K resistor when the EQ switch is in the IEC position.  Played with adding resistance to the 1.06K and found that an additional 220 ohms upped the HF response so it is now within a dB at 10K and down 2 dB at 20Khz. 

Close enough for my needs but as I don't have the owners deck I have no idea what the "net" frequency response is with that combination, so I'll just return it to "stock" before sending it back.  Even if he has a test tape and knows how to use it, how are he (and Jay) supposed to correct an imbalance?       

Charles

[docb]

Back in the office Monday morning to quite a little tempest, I see.

Charles, we build Tube Repros for specific customer applications. Most of our customers have no desire nor the equipment to make the kind of adjustments you are talking about here. We usually set up the machine and the Repro for them here at our lab. If you would like to purchase a Tube Repro that is built to your requirements please contact me. We can do and have done some with trim in the eq (ironbut has one) and others with trim at the input to allow for head/cable interactions.

In a sense it's a shame that you borrowed a Tube Repro and went inside it like that because now that person has lost part of what they paid $4000 for, that being the warranty.

Jay, it's not clear to me - are you listening to a TP tape on the NAB playback setting and hearing too much treble? That would indeed be the case. If you are hearing it on an NAB tape let me know and we can walk through the steps to determine what is causing what you are hearing.

[jcmusic]

Jay, it's not clear to me - are you listening to a TP tape on the NAB playback setting and hearing too much treble? That would indeed be the case. If you are hearing it on an NAB tape let me know and we can walk through the steps to determine what is causing what you are hearing. 
 
Indeed Doc,
That is the case, I am hearing an almost unlistenable  HF with a NAB tape in the NAB position.

jay

[docb]

OK, the way the circuit works the treble corner frequency is set for NAB by one resistor and it is adjusted up for IEC by putting another resistor in parallel. So in theory if the resistor value we spec'd for NAB sounds too bright because the resistor value is off the IEC would probably sound too bright as well.

There are other possible sources for this kind of problem, so bear with me while I try to eliminate a few possibilities to close in on it. Are you listening to the same head for both NAB and IEC, or are you listening to NAB on the 1/4 track head and IEC on the two track head?

[jcmusic]

Yes that is correct Doc, NAB 4 track and IEC 2 track.

Jay

[docb]

Since the problem seems to appear when you use the 1/4 track head it could possibly be the alignment of the 1/4 track head that is affecting the sound. IME 1/4 track heads are often fairly touchy to get just right. Unfortunately there aren't many 1/4 track alignment tapes around. We align visually with clear tape, then take a first cut with a two track tape, and then Shawn and I usually throw on a 1/4 track tape of music we know really well to do the final touch up on alignment. Being off just a little bit laterally can throw the frequency response off quite a bit - it might lose bass, it might lose treble, you can also hear the stereo image get better and worse. You might want to try listening with headphones when you touch up the alignment this way. Go slow, and pay special attention to how much you turn each screw so you can return it to where you started if it doesn't improve things, before you turn the next screw.

Now I will say the IME it's usually the top end that goes first when alignment is off rather than the bottom end, but it won't hurt anything to try this and see if it makes a difference.

Once you have established that the 1/4 track alignment is optimal we can dig a little deeper, if necessary.

[jcmusic]

Ok Doc,
Thanks for the tips, but I am not so sure the alignment is off. All though it could be and you are correct, I say this because I just had the heads relaped by John French and said the alignment was be as close to perfect as one could get without the use of a scope.
With that in mind I am thinking that it could quite possibly be the tape in question. I will have to play some other tapes to find out, thanks for your help just the same.

Jay

[docb]

Let me know how it goes. The variability of 1/4 track prerecords is extreme, to be sure.

[ironbut]

I've been spending a lot of time doing a survey of levels and recording qualities lately (I'm about 50 tapes into it so far). These are all classical 1/4 track tapes. For the most part, I've found that the sound quality has been really good (aside from a few moments where the sound fuzzes out on extreme peaks). But one thing that I've noticed is that with smaller ensembles such as string quartets, the violins can get screechy and hard sounding (almost like digital). Unfortunately, I love string quartets and although I've only come across two that have this problem, I think it must be something to do with the recording practices at that time. I'm guessing that they may have "pushed" the tape a little harder to increase the S/N ratio and push the hiss further down or added a little high frequency boost to add air to the recording. Remember that nobody had gear that was as resolving as we do now so errors in translation (adapting a recording to sound good on a variety of home equipment) might have seemed like the right thing to do at the time.

[jcmusic]

Yea maybe so Steve, you are on to something. Like I said to Doc it could very well be the said tape in question, I will try some others and see what the results are. Thanks for your imput.

Jay