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TapeHead Tube Preamp Schematic Question

dromichet

Member
2021-11-04 10:40 pm
Thank you very much. I will load it up, make sure I have the necessary tube stuff and play around with it.

For testing purposes, to ensure the PS is not the problem, I've connected the 2 stages to another PS from a working vintage tube amp. The results are the same. With only first stage connected I get about 30db gain. The moment I connect the second stage to the first stage, the first stage gain drops to 15db gain. The second stage adds a gain of 30db, about 45db in total. It should have been about 60db total in gain.

The first 2 stages are very simple to build and I can't see any problem with my work. The values should be good as your sim shows it should be fine.

Is it normal for the first stage gain to drop by half the moment a second stage is added?

I'm starting to doubt myself even though I've build much more complex tube circuits before.
 
Without power, measure the resistance at that V5B grid to ground.
Another way, with power, apply signal generator directly to the disconnected V5B grid through an approximately 100kohm resistor. Measure signal on the signal generator side and the grid side of the resistor. (V5B grid not connected to first stage). Report signal at both points. Attenuation should be less than 20%.
 

dromichet

Member
2021-11-04 10:40 pm
It looks like it's working now but I can't tell you why. The circuit is build on a circuit board however the PS part is on a breadboard. All the plate and grid voltages were correct as in the original schematic yet the signal was not amplified properly.

I even connected a different PS but using the same RC filters and was the same crappy gain. But when I connected the SMPS back on it all started to work properly. I checked the plate and grid voltages and they're the same as the time when it didn't work. Could it be that a bad connection caused less current draw? I don't know enough to figure this one out.

I feel like I wasted a lot of your time and I must apologize. I'm embarrassed although I have learned a lot from this troubleshooting and I thank you.

I'm still confused why it all started to work on its own and I fear tomorrow will stop again and still don't know why...lol.

My next step is to finish the second channel and see how the FR will be when hooked up to an actual tape head.

Thanks again for all your help.
 
The original schematic was based on the Tandberg 64X reel-to-reel schematic.

Maybe this is a stupid question, but is it possible to adapt that circuit for use as a MM phono preamplifier, adapting some passive values for RIAA curve equalisation, or is the equalisation for tape too deeply embedded in the design of the circuit?
 

dromichet

Member
2021-11-04 10:40 pm
Thank you very much Potentiallyincorrect. You’ve been a lot of help with my project. I still have a lot to go on this one. Having eq combined with a tape head adds a bit of complexity so the ltspice is probably the way to go forward.

Without the tapehead and just with a signal generator at its input, the low end should be boosted by about 20db. Your sim confirms that to be correct however my actual build only boosts it by about 9db. From 1k up is all good so I’ll play around and figure out what’s diferent between the sim and actual build.
 

dromichet

Member
2021-11-04 10:40 pm
OldHector, nab and riaa are very similar and many people and companies have modified a riaa eq to a nab eq. Many tube vintage receivers have a head input as well as phono input. When you look at the schematic you’ll see that the tape input(nab eq) is actualy a modification of phono input (riaa eq).

If you’re after building a tube phono amp there are plenty of schematics online, much better ones than the Tandberg 64x.
 

dromichet

Member
2021-11-04 10:40 pm
I got LTspice working and was able to run simulations. The simulation FR curve matches the NAB curve which is a good thing.

However, when I connect my iphone (signal generator) to the amp built, the lower freq boost is only by about 7db above 1KHz (0db reference) instead of about 20db as the sim and NAB curve. Connecting it to a Tandberg TD20A tape head it also produces a very thin sound matching the signal generator.

I understand that the newer tape heads like the Tandberg TD20A are low inductance heads build for SS. Older tape machines used high inductance repro heads which were build for tube preamps like the Tandberg 64x which this schematic is based on.

Could it be that the impedance between the "modern" tape head and the older tube tape head preamp are not matching causing it to lose all its bass? Is there a way to make up for it if that's the case?

Or is there something else wrong?

Otherwise the preamp sounds good with the higher freq but it has no bass.
 

dromichet

Member
2021-11-04 10:40 pm
Thanks, I will look into the ECC808 tube. Do you know if I need to alter other components to fit this tube into this schematic?

I was also thinking of trying to fit a jfet as the input before the tubes. I see the following model of Tandberg 74b used a transistor as the input followed by tubes. I'm wondering if the did that because they switched from high to low inductance heads.
 

Sorin Cristea

Member
2021-02-24 11:47 am
Thanks, I will look into the ECC808 tube. Do you know if I need to alter other components to fit this tube into this schematic?
Electrically,the 808 is a ECC83.Quiet as a mouse (if I recall corectly,quieter than most NOS 83's...think about having a ECC803S but better,at a fraction of a cost).I use the 808 tube in the first position of a RIAA preamp designed for 83/AX7/7025 with much better results and the only mods I made were the socket rewiring.
Regarding the other issues you asked,a jFet before the tube might get you even quieter behaviour at the expense of an aventually "loss of the tube flavour" (not a bad thing "per se",it all depends of your level of tolerance ;)