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Revox B 36 tape recorder banana plug output

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Revox B 36 tape recorder banana plug output

Postby timoc » Sun Mar 08, 2020 2:12 pm

Hi Everyone,

I've been given a Revox B36 mono tape recorder by a family member and would like to record the sound out with a line.

There is one output, which is intended for an external speaker and requires a 'banana plug'. The handbook states that the connection is 'intended for a 5-10 ohm external loudspeaker' but that the 'output can be fed into pre-amplifiers'.

I have found one place that sells a cable with rca plugs on one end and banana plugs on the other (https://www.amazon.de/FidgetGear-Banana-Adapter-Stereo-Verst%C3%A4rker-Receiver/dp/B07VGL38QC/ref=sr_1_fkmr2_1?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&keywords=RCA+Phono+To+Banana+Plug+Adaptor&qid=1583516436&sr=8-1-fkmr2).

Will this allow me to record into my soundcard or mixer, or are the ohms a problem?

Thank you,
Timo
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Re: Revox B 36 tape recorder banana plug output

Postby MOF » Sun Mar 08, 2020 2:33 pm

You should be fine with the impedance, you need to go from a low source to a higher destination impedance.
Your problem will be preventing distortion from the output being too high for a line level input. If there’s an output level control on the B36 start at the lowest level and increase it slowly to the point where it’s at a good level but not distorting.
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Re: Revox B 36 tape recorder banana plug output

Postby timoc » Sun Mar 08, 2020 2:39 pm

MOF wrote:You should be fine with the impedance, you need to go from a low source to a higher destination impedance.
Your problem will be preventing distortion from the output being too high for a line level input. If there’s an output level control on the B36 start at the lowest level and increase it slowly to the point where it’s at a good level but not distorting.

Thank you for the quick reply!

What about the other way around? There are three inputs:
1. Microphone input, high impendance, sensitivity 2.5 mV at 2 megohm for crystal or dynamic microphone with transformer
2. Radio input, high impendance, sensitivity at 300mV at 2 megohm
3. Radio input, high impendance, sensitivity adjustable from 10 - 500 mV

Can I use the same cable to record onto the tape, and which input would I use?

Thanks again, Timo
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Re: Revox B 36 tape recorder banana plug output

Postby ef37a » Sun Mar 08, 2020 3:02 pm

That is a speaker output and I would load it with 8 Ohms 10 watts if you want to get the signal to a recorder.

However, the B36 machines I have found online have a line output of 0.7V i.e. about 0dBu and would give better quality. These outs are said to require a load of not LESS than 100k so the high impedance input of an interface would be the best option.

Dave.
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Re: Revox B 36 tape recorder banana plug output

Postby James Perrett » Sun Mar 08, 2020 3:18 pm

MOF wrote:You should be fine with the impedance, you need to go from a low source to a higher destination impedance.

This is a valve recorder so the speaker output would normally need to be loaded correctly to avoid damage to the output transformer. However the manual specifically states that this output can be used to feed an external amplifier so there must be suitable protection in this case.

You can find a manual at https://www.hifiengine.com/manual_libra ... /b36.shtml.
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Re: Revox B 36 tape recorder banana plug output

Postby Trevor Johnson » Sun Mar 08, 2020 4:47 pm

I note that the final audio amp is an EL84, hence the B36 output is rated at 3.5W. And in 1957, or thereabouts, the preamplifier would have been valved, which may have a bearing on things.

For example, a high impedance input to the grid of whatever valve is used as the first stage of amplification in the preamp.
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Re: Revox B 36 tape recorder banana plug output

Postby Hugh Robjohns » Sun Mar 08, 2020 7:01 pm

timoc wrote:Will this allow me to record into my soundcard or mixer, or are the ohms a problem?

Yes, that should be fine. Just make sure you plug the black banana plug into the grounded outlet (on the right, I think).

The impedance isn't likely to be a problem with a standard standard line input -- but a high-impedance instrument input might be better. Just be careful with the levels and obviously, start with the tape machine's volume control fully down and raise it cautiously!

The output stage (see below) has a feedback path from the transformer's secondary which is intended to compensate for the actual output load (speaker or preamp)

B36.png


H
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Re: Revox B 36 tape recorder banana plug output

Postby ef37a » Sun Mar 08, 2020 7:06 pm

Trevor Johnson wrote:I note that the final audio amp is an EL84, hence the B36 output is rated at 3.5W. And in 1957, or thereabouts, the preamplifier would have been valved, which may have a bearing on things.

For example, a high impedance input to the grid of whatever valve is used as the first stage of amplification in the preamp.

My last post has not appeared. This happens now an again with Soz but never I don't think with the two other forums I regularly infest?

I was going to say that the 'protection' for the valve output stage seems to be that the 4mm plug is only pushed in part way and thus the internal speaker still works. Not a very satisfactory arrangement to my mind and one that could lead to damage? Personally I would not want to risk the OP traff in that venerable old Revox.

A 6r8 'Allyclad' resistor costs about 3 quid at CPC and and a metal box about a fiver. Add input and output jacks, a small amount of sweat and you have a load box for under a tenner?

Or, get a tech to fit a pre VC line feed.

Dave.
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Re: Revox B 36 tape recorder banana plug output

Postby Hugh Robjohns » Sun Mar 08, 2020 7:15 pm

ef37a wrote:My last post has not appeared. This happens now an again with Soz...

I don't know how many posts you've tried to make in this thread, Dave ... but there's definitely one more above this one.

I'm posting on the forums all day and I never have problems with missing posts... but what can sometimes happen is that someone else posts while you're typing and then when you hit the Submit button the forum software automatically holds your post in limbo and presents a warning page about the other post... This is intended to allow you to change your post before submitting, in response to the new one.

I believe this can confuse some people who don't realise that (despite hitting Submit once already) their post hasn't actually been published yet... and if you don't click Submit on that holding screen your post will be lost to the ether.
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Re: Revox B 36 tape recorder banana plug output

Postby ef37a » Sun Mar 08, 2020 7:30 pm

Hugh Robjohns wrote:
ef37a wrote:My last post has not appeared. This happens now an again with Soz...

I don't know how many posts you've tried to make in this thread, Dave ... but there's definitely one more above this one.

I'm posting on the forums all day and I never have problems with missing posts... but what can sometimes happen is that someone else posts while you're typing and then when you hit the Submit button the forum software automatically holds your post in limbo and presents a warning page about the other post... This is intended to allow you to change your post before submitting, in response to the new one.

I believe this can confuse some people who don't realise that (despite hitting Submit once already) their post hasn't actually been published yet... and if you don't click Submit on that holding screen your post will be lost to the ether.

Thanks Hugh. I am no longer confused by the 'hold the presses' function, just annoyed!
I am sure you are right and this old duffer has poked/not poked wrongly.

That is one weird output stage! They are putting the cathode current through the transformer secondary ( rather like Quad or Mackintosh. I wonder who got there first?).
I still don't trust it for such an old machine. I would get a tech to take a signal from the 2k2 in the cathode of the second triode stage.

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Re: Revox B 36 tape recorder banana plug output

Postby Trevor Johnson » Sun Mar 08, 2020 7:32 pm

And it's a B36, so has 260V DC, on the anode, across which is the output transformer.
Sensible to have it checked out for electrical safety, first, before proceeding.

Unless you are equipped to do so.
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Re: Revox B 36 tape recorder banana plug output

Postby Trevor Johnson » Sun Mar 08, 2020 7:45 pm

That is one weird output stage

Dave, I agree, I wondered what is going on here! But my valve knowledge is mainly RF with a bit of 'light' audio at the end point!
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Re: Revox B 36 tape recorder banana plug output

Postby MOF » Sun Mar 08, 2020 8:30 pm

I'm posting on the forums all day and I never have problems with missing posts... but what can sometimes happen is that someone else posts while you're typing and then when you hit the Submit button the forum software automatically holds your post in limbo and presents a warning page about the other post... This is intended to allow you to change your post before submitting, in response to the new one.

I also once used an iPhone emoji instead of the SOS supplied ones and that didn’t work.
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Re: Revox B 36 tape recorder banana plug output

Postby timoc » Sun Mar 08, 2020 9:22 pm

ef37a wrote:I was going to say that the 'protection' for the valve output stage seems to be that the 4mm plug is only pushed in part way and thus the internal speaker still works. Not a very satisfactory arrangement to my mind and one that could lead to damage? Personally I would not want to risk the OP traff in that venerable old Revox.

A 6r8 'Allyclad' resistor costs about 3 quid at CPC and and a metal box about a fiver. Add input and output jacks, a small amount of sweat and you have a load box for under a tenner?

Or, get a tech to fit a pre VC line feed.

Dave.

Thanks, I may have to do the latter, given I don't know how to make a load box.

I think the manual states that if you've pushed the banana plug all the way in and still hear the original speaker, you can push a second plug into the left adjacent socket halfway to kill the speaker sound:

Image

What could go wrong with the method described by others of plugin in a banana - rca cable and turning up the volume slowly?
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Re: Revox B 36 tape recorder banana plug output

Postby James Perrett » Sun Mar 08, 2020 9:35 pm

timoc wrote:Thanks, I may have to do the latter, given I don't know how to make a load box.

I think we've worked out that you don't need a load box with this particular design of output. Just do what you originally intended and use a couple of banana plugs - they're easily available from most component suppliers or on Ebay.
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