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Old 12th Sep 2011, 11:12 pm   #1
qualityten
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Default Mono amplifier transformers BBC?

I couldn't resist buying this pair of transformers which a vendor had removed from an amp. He couldn't remember anything about the amplifier except that it had failed its PAT test and was called "BBC2 something". He had sold the valves separately, and it may have used a pair of KT66s.

Do these transformers look familiar to anyone? I'd like to use them for a winter mono project. They are huge and look good quality. From the markings they look as if they might have been specially made for a particular application.

I'd be grateful for any hints on where I might look for more information.
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Old 12th Sep 2011, 11:34 pm   #2
GrimJosef
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Default Re: Mono amplifier transformers BBC?

They look a lot like the transformers in my Grampian PA amps, even down to the hand-painted labels. Those amps use pairs of KT66s. I'll see if I can find some pictures.

Cheers,

GJ
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Old 12th Sep 2011, 11:56 pm   #3
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Default Re: Mono amplifier transformers BBC?

There are some pics of Grampian transformers (from a slightly smaller amp, I think) in the first post of this thread https://www.vintage-radio.net/forum/...ad.php?t=69136. And I've attached one of my Grampian 602. This only shows the cores and windings, not the tags and labels, but I think they are very similar to yours. The 602 was rated at 25-35W for PA use.

Cheers,

GJ

Edit: Actually I've just checked the schematic for my 602 and the output transformer is labelled OT72 so it may well be smaller than yours. Grampian made 50W amps using a pair of KT66s (types 501 and 600) and even a 100W unit with four KT66s (type 531, although this only had a 100V line output).
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Old 13th Sep 2011, 5:33 am   #4
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Default Re: Mono amplifier transformers BBC?

Thanks GJ. The closeup of the voltage selector patch at the bottom of this page is similar to the one attached to my power transformer.

I found this pic of a Grampian 600U, which has transformers of a more similar size. Mine may be from a 501. It would be great to locate a circuit diagram.
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Old 13th Sep 2011, 8:12 am   #5
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Default Re: Mono amplifier transformers BBC?

Circuit diagrams for Grampians seem to be hard to come by. In the end I had to trace out the one for my 602. I've got the little sales leaflets for many of them but all these have are a small picture of the amp itself, a list of the technical specs and valve complement and some words about the ruggedness of the steel case and the possible uses the amp might be put to.

In fact you probably don't want to reproduce the Grampian circuit. PA amplifiers were generally designed to deliver a lot of power over a relatively narrow frequency band. It seems that extending the frequency range tends to make speech less intelligible in a noisy environment. Relatively large levels of amp noise and distortion also don't matter very much. So the type 600 amp is specified as delivering 50W over a frequency range of 100Hz-10kHz with less than 5% distortion. Hi-fi it ain't !

Cheers,

GJ
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Old 13th Sep 2011, 8:46 am   #6
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Default Re: Mono amplifier transformers BBC?

I have a lot of BBC TIs. Loudspeaker amps are in the AM8 range so any hint of a number might help me locate a TI. There are earlier BBC numbering systems so it might begin LSM. Unfortunately TIs rarely if ever have photos or even physical layout drawings.

Just leafing through my TIs I found a full Designs Dept spec for the AM8/11 which turns out to be the BBC version of the Quad 50D.

A couple of the BBC AM8/xxx amps use a pair of EL34 in ultralinear so these could be candidates.
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Old 13th Sep 2011, 9:49 am   #7
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Default Re: Mono amplifier transformers BBC?

Quote:
Originally Posted by GrimJosef View Post
In fact you probably don't want to reproduce the Grampian circuit. PA amplifiers were generally designed to deliver a lot of power over a relatively narrow frequency band. It seems that extending the frequency range tends to make speech less intelligible in a noisy environment. Relatively large levels of amp noise and distortion also don't matter very much. So the type 600 amp is specified as delivering 50W over a frequency range of 100Hz-10kHz with less than 5% distortion. Hi-fi it ain't !
That's nice in a way. Given that someone else has already demolished the rest of the amp, these transformers are free to be used any way. The power transformer would work with a Mullard 5-20, which I have learned about with the help of this forum, but I'd like to investigate the capability of the output transformer.

I know nothing about BBC TIs, so that will be interesting to investigate too. ppppenguin, do these transformers resemble the BBC TIs you know? Might Grampian have supplied the BBC?

Last edited by qualityten; 13th Sep 2011 at 9:50 am. Reason: Getting the right number of ps for ppppenguin's name.
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Old 13th Sep 2011, 9:58 am   #8
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Default Re: Mono amplifier transformers BBC?

Since there are no photos or mech drawings in the TIs it's difficult to judge. It is likely that Grampian supplied the BBC but I have no specific knowledge.
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Old 13th Sep 2011, 10:39 am   #9
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Default Re: Mono amplifier transformers BBC?

Quote:
Originally Posted by qualityten View Post
The power transformer would work with a Mullard 5-20, which I have learned about with the help of this forum, but I'd like to investigate the capability of the output transformer.
The most effective way to do that, to start with at least, would be to make some measurements on it. The simplest thing to establish is the turns ratios. I suspect there are three windings: the primary (centre-tapped and with internediate taps for ultralinear operation), the low impedance secondary (with a couple of tappings for different speaker impedances) and the high impedance secondary (for driving 100V line speakers often used in PA applications). To get the turns ratio apply a small AC signal (just a few volts) between the ends of the low impedance secondary (if you're in doubt you may be able to confirm which are the end connections by making resistance measurements, but the resistances will probably be pretty low). Then measure the voltages across the other windings. Careful - there might be a couple of hundred volts here. The voltage ratios are equal to the turns ratios.

Once you know the turns ratio between the primary and, say, the 7.5 ohm secondary you can work out the anode-to-anode impedance of the primary. It's just 7.5 ohms multiplied by the square of the turns ratio. The a-a impedance will narrow down your choice of output valves and knowing that, and the ultralinear tapping percentage (which may be 25% or 43%), you can start considering particular designs.

One thing I can see is that the connections to what looks like the high impedance secondary are marked 400-CT-400. I'm not clear exactly what this means but if it's saying that the intended load between the ends of this winding is 400 ohms then that suggests the amp was designed to deliver 25W (power is the square of the voltage divided by the load resistance which is 10,000/400=25 assuming that the winding is for a 100V line output). If you can post the physical dimensions of the iron core (not the windings) that could also indicate the power capability.

Cheers,

GJ
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Old 13th Sep 2011, 11:31 am   #10
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Default Re: Mono amplifier transformers BBC?

Be aware that several BBC loudspeaker amps were designed for unusual impedance speakers. 25R rings a bell, without going back to my TIs and checking.

PS: Just checked. Both AM8/1 and AM8/4 are 15W into 25R. 2x EL34p-p ultralinear.

PPS: I have schematic only for LSM8 which is 2x KT66 p-p. Looks like triode connected, just a resistor between anode and screen grid.

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Old 13th Sep 2011, 6:41 pm   #11
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Default Re: Mono amplifier transformers BBC?

Thanks for the information and suggestions thus far. Jeffrey, I'd be most interested in seeing the schematic for the LSM8, please. I'll PM you.
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Old 13th Sep 2011, 8:42 pm   #12
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Default Re: Mono amplifier transformers BBC?

It's a 602D. 25watts, possibly a bit more using the 440v taps
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Old 13th Sep 2011, 9:06 pm   #13
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Default Re: Mono amplifier transformers BBC?

Thank you Bill, I see now that the power transformer is marked P7B. Thank you too for these fantastic pictures of the circuit diagram--I can almost feel the texture of the paper.

So, the question is, given that I am more interested in hifi than PA, would it be worth building this circuit? Would GJ's verdict 'hifi it ain't' apply to this amp too?
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Old 13th Sep 2011, 9:48 pm   #14
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Default Re: Mono amplifier transformers BBC?

'Fraid so. As you can see from the flyer, I hope, the frequency response and distortion (50Hz-10kHz +/-1dB and 2% @ 25W) are better than the larger units but are still not wonderful. At least part of the problem is that the level of negative feedback is not very high, presumably to avoid reducing the gain too far but perhaps also because the output transformer may contribute significant phase shift towards the frequency extremes ? The feedback loop also doesn't include the first two stages. It can't, of course, because otherwise it would cancel the effects of the tone controls which are located after the second stage.

Cheers,

GJ
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Old 13th Sep 2011, 10:04 pm   #15
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Default Re: Mono amplifier transformers BBC?

This is really interesting. I'm pretty sure that it would offer a more enjoyable 25W than some of the solid state PAs I've endured! Interestingly, I see on the circuit that the OT80 is a modification (upgrade?) of the OT72 output.

A couple more questions, if I may. What does the CZ1 from the CT of the power transformer (cap or resistor with a connection) represent?

Is the stated frequency response a result of the circuit design, and intended application? Would removing the tone controls improve matters or might a different KT66 circuit produce better fidelity?
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Old 13th Sep 2011, 10:35 pm   #16
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Default Re: Mono amplifier transformers BBC?

The CZ1 is a thermistor. It has a relatively high resistance when cold so it limits the initial current which flows in the HT circuit thereby protecting other components from any rush of current at switch-on. Once it's warmed up its resistance falls to the point where it no longer has much of an effect.

I imagine that the frequency response is a consequence of the design but whether it can be improved much depends, in the end, on the quality of the transformer and on how much gain is needed. Removing the tone controls would help since they are inevitably lossy and since they also prevent the application of full negative feedback. But in the end the transformer will set the limit. There are ways of measuring its performance to establish what that limit will be but I fear they're not trivial to carry out. Basic ones include primary inductance (should be as high as possible) and leakage inductance (should be as low as possible). Those more expert than me can no doubt tell you about others.

Cheers,

GJ
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Old 14th Sep 2011, 9:13 am   #17
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Default Re: Mono amplifier transformers BBC?

Thanks for clarifying that the CZ1 is a thermistor. How do I read the value, presumably specified at room temperature?
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Old 14th Sep 2011, 9:50 am   #18
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Default Re: Mono amplifier transformers BBC?

Some data for the CZ1, and others, here http://www.domino405.co.uk/documents/Brimistors.pdf. If you want to check that yours is in spec you can just measure it with a multimeter as with any other resistor.

Cheers,

GJ
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Old 14th Sep 2011, 4:53 pm   #19
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Default Re: Mono amplifier transformers BBC?

Thanks. A very helpful overview of the Brimistor brand.
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