Making old Ampex tube reel to reel play nice with audio interface?

[AardvarkBry]

I have an old tube reel to reel from 1959 that I restored. It sounds amazing when I hook it up to my old Dynaco Pam-1 to Dynakit Mark iii, but when I try to use it with my audio interface and DAW, things get really problematic. Sending a line out from the Apollo to the line in of the Ampex, I have to boost the gain to almost max to get any signal. If I send the line to the mic inputs, it clips like crazy. When I send a line out back to the Apollo, again, I have to boost it to max, and it still doesn't get a ton of signal. The whole operation adds up to a ton of hiss.

Since the Ampex is all unbalanced, I built a passive direct box with 600ohm:600ohm transformers. It helped the gain a little bit, but I lose a ton of low end bandwidth.

Looking at the circuit, the preamp doesn't have any grid stoppers, cathode bypass caps, negative feedback, or input or output transformers. The output is a cathode follower, which I know very little about. I assume some combination of adding these things can help with gain, and noise. can someone give me any mod advice?

 

[radardoug]

Have you got a scope? It is likely the unit does standard line levels, so you must be doing something wrong. A scope would help you find out.

 

[kbx]

How were you able to get any gain with the same primary & secondary on the transformer?

 

[AardvarkBry]

Have you got a scope? It is likely the unit does standard line levels, so you must be doing something wrong. A scope would help you find out.

I don’t have a scope. I checked all the biases and voltages against their chart, and everything looked normal. The weird thing is, It sounds spectacular running through my old tube preamp, and it sounds pretty decent with the Apollo, except that gain levels aren’t really useable. The old manual for the unit says it has a “high-impedance line input (radio, phono, aux, tv) .3 volt rms for program level, and high impedance microphones. Playback level 1/2 volt rms from cathode follower when program tapes are being played.”

 

[AardvarkBry]

How were you able to get any gain with the same primary & secondary on the transformer?

I just used a standard 1:1 line balancing transformer, just to convert the balanced line out of the Apollo, to the unbalanced line input of the r2r.

 

[AardvarkBry]

How were you able to get any gain with the same primary & secondary on the transformer?

I thought about using a step up transformer, but I don’t really know what input impedance the unit is expecting? There is no grid stopper resister, or transformer on the input, and from my understanding, a 12ax7 without anything to set the impedance can be up over 1meg ohm

 

[kbx]

I would try running a 60Hz* tone through the unit and measure the AC at the output jacks with your DMM. This will at least confirm that your unit is putting out the correct level.

*edit: Depending on what your DMM can read accurately, something closer to a 1k tone is preferable.

 

[Brian Roth]

Those Olde Ampex consumer machines were designed for a -10 dbV universe. I don't have a clue why you have to crank up the level from your Apollo to get a decent level into the Ampex input. A pro level signal into that RCA jack on the Ampex RCA jack will blast the doors of that line level RCA input.

I CAN see why the output from the Ampex back into the Apollo will be "weak". In that case you need a bump box. -10 dBV to +4 dBu.

https://www.aphex.com/products/124b
Bri

 

[ruffrecords]

You don't need grid stoppers on low mu tubes like the ECC82 and ECC83. The amp has lots of negative feedback. There is 100% around the cathode follower and there is NFB from the second half of the ECC83 to the first half to achieve the required playback equalisation.

Although the cathode follower provides low output impedance, the stage is running at about 2mA quiescent which means its drive capability is limited. These machines were not designed to drive modern 10K loads. If your Apollo has a DI input try using that otherwise try buffering it with a DI box.

Cheers

Ian

 

[AardvarkBry]

You don't need grid stoppers on low mu tubes like the ECC82 and ECC83. The amp has lots of negative feedback. There is 100% around the cathode follower and there is NFB from the second half of the ECC83 to the first half to achieve the required playback equalisation.

Although the cathode follower provides low output impedance, the stage is running at about 2mA quiescent which means its drive capability is limited. These machines were not designed to drive modern 10K loads. If your Apollo has a DI input try using that otherwise try buffering it with a DI box.

Cheers

Ian

I was looking at an Ampex 351 schematic, and it is surprisingly similar to this consumer machine. The only real power difference is, the playback preamp has 1 extra 12ax7 tube, both plates, and the output uses both 12au7 plates push pull, and there’s a 12at7 as the mic pre output, instead of the 12au7. Also, there’s an input mic transformer, and a 15k/600/nfb output transformer. The power supplies both look very similar, with 6x4 rectifiers, and 2 separate heater supplies, 1 DC, and 1 AC. Do you think if I just copied the 351 input, and output section, I’d have enough power to handle it?

 

[AardvarkBry]

Those Olde Ampex consumer machines were designed for a -10 dbV universe. I don't have a clue why you have to crank up the level from your Apollo to get a decent level into the Ampex input. A pro level signal into that RCA jack on the Ampex RCA jack will blast the doors of that line level RCA input.

I CAN see why the output from the Ampex back into the Apollo will be "weak". In that case you need a bump box. -10 dBV to +4 dBu.

https://www.aphex.com/products/124b
Bri

Thank you. I have an old Symetrix 501, with a -10dBV to +4 switch. I’ll try to run through that, and see what happens.

Do you know what kind of source impedance the mic pres are expecting? It just says high-impedance microphone, but that could mean almost anything. Are we talking like 50k, or like a 1meg crystal mic? I’ve tried every mic I have, and they all blast its doors off, and clip like crazy. I built a little box with a 600:50k transformer. I’ve tried a bunch of different pres. I tried plugging a guitar into it. They all clip. Maybe I’ll try a piezo, and see what happens.

 

[kbx]

Sorry, can you explain the signal chain again? I'm unclear on what's going in/out/in of which machine and what it's being used for.

 

[ruffrecords]

I was looking at an Ampex 351 schematic, and it is surprisingly similar to this consumer machine. The only real power difference is, the playback preamp has 1 extra 12ax7 tube, both plates, and the output uses both 12au7 plates push pull, and there’s a 12at7 as the mic pre output, instead of the 12au7. Also, there’s an input mic transformer, and a 15k/600/nfb output transformer. The power supplies both look very similar, with 6x4 rectifiers, and 2 separate heater supplies, 1 DC, and 1 AC. Do you think if I just copied the 351 input, and output section, I’d have enough power to handle it?

You problem is going to be the mains transformer. I am pretty certain it will not be able to supply the extra heater power required.

Cheers

Ian

 

[inkster]

If you want to use a transformer on the front end a quality 10k:10k is what I use with my 350(or 351 when I had one), that way you can use the unbalanced 350/351 input and bypass the high gain(noise) mic pre stage which is similar to using the line input on your machine.
I would also check the apollo to see if it is actually truly balanced.

 

[rmburrow]

I have an old tube reel to reel from 1959 that I restored. It sounds amazing when I hook it up to my old Dynaco Pam-1 to Dynakit Mark iii, but when I try to use it with my audio interface and DAW, things get really problematic. Sending a line out from the Apollo to the line in of the Ampex, I have to boost the gain to almost max to get any signal. If I send the line to the mic inputs, it clips like crazy. When I send a line out back to the Apollo, again, I have to boost it to max, and it still doesn't get a ton of signal. The whole operation adds up to a ton of hiss.

Since the Ampex is all unbalanced, I built a passive direct box with 600ohm:600ohm transformers. It helped the gain a little bit, but I lose a ton of low end bandwidth.

Looking at the circuit, the preamp doesn't have any grid stoppers, cathode bypass caps, negative feedback, or input or output transformers. The output is a cathode follower, which I know very little about. I assume some combination of adding these things can help with gain, and noise. can someone give me any mod advice?

I have an old Ampex F-4460, same situation. You need high impedance transformers on the L & R outputs of the Ampex like a UTC A24 or equivalent. Connect the 600 ohm secondary to the DAW input. Ground the transformer cases to the DAW which receives the signal from the Ampex. You will then need to adjust levels as necessary . The high impedance cathode follower output of the Ampex needs to see a high impedance transformer primary to talk to pro devices or the outside world.

 

[AardvarkBry]

I have an old Ampex F-4460, same situation. You need high impedance transformers on the L & R outputs of the Ampex like a UTC A24 or equivalent. Connect the 600 ohm secondary to the DAW input. Ground the transformer cases to the DAW which receives the signal from the Ampex. You will then need to adjust levels as necessary . The high impedance cathode follower output of the Ampex needs to see a high impedance transformer primary to talk to pro devices or the outside world.

Nice, so a 15k:600 does the trick? I have a ton of 600:50k transformers, but I assume hooking one of those up backwards is probably overkill right?

Probably a dumb question, but I’m confused by what you mean ground the transformer case to the DAW? Are you saying to attach a ground wire to the TX box and attach the other end to pin 1 of the XLR output?

 

[rmburrow]

Make the transformer's 50 k winding the primary and the 600 ohm winding the secondary. Ground the transformer case (and any transformer ground connection) to pin 1 of the XLR going to the DAW. This is to minimize risk of a ground loop. I am assuming the DAW XLR input is balanced, so the 600 ohm winding should keep the DAW happy.

Reversing the primary and secondary should not affect the audio adversely; a transformer is a transformer and the turns ratio is fixed. The 50k primary may be a better load for the unbalanced HiZ output of the cathode follower since the coupling capacitor in the Ampex is 0.1 mFd off the cathode.