GEM SDR 50R - EL84 or 7189A final tubes in this circuit ??? some clues

[kagliostro]

I finished my reverse engineering draw of the GEM Super Deluxe Reverb 50 R amp

Hope someone can help to revise

Some clues make me think that the el84 tube that were mounted on the amp weren't the correct tubes

I draw the schematic with the standard el84 pin connections

indeed the connection were a bit different, initially I was assuming that they used some unused pin to attach components

but now I'm thinking the right tube for this circuits were 7189A or 7189 tubes

I am inclined to think that for this reasons:

The pin connection on the final tubes

The High Voltage (for el84 tubes) of the PT (at 220v input corresponds AC 366v-0-366v & AC 7v heater)

In one italian forum I've read an old thread where someone told he got one of that amps and he refer to the amp as a 50W amp

The 7189A tube has connections and plate voltage ratings that seems agree with the circuit

What do you think about ?

----

PRR in one other thread explained some about the Varistor Vibrato circuit

The Varistor capacitance changes with voltage. If V3b is driven hard with a low frequency, the Varistors will var the gain and tone of the signal at V3a.

But I'm not able to understand in detail how this acts ( ?? )

THANKS

Kagliostro

attached the full schematic and the schematic of the real connection to the power tube

[kagliostro]

Here a bit documentation about the el84 & 7189A tubes

http://www.nj7p.org/Tube4.php?tube=EL84

http://www.nj7p.org/Tube4.php?tube=7189A

and this very interesting thread at Ampage archive

http://archive.ampage.org/threads/1/gadc/028895/Wiring_up_EL84s-1.html

read what wrote MBSetzer in 06/06/2000


can someone confirm my supposition ?

Thanks

Kagliostro

[sluckey]

I'm a little confused by this drawing...



If pin 2 has no connection, then you absolutely cannot use a 6BQ5, EL84, or 7189 tube since pin 2 is the control grid. However, you can use a 7189A because the control grid is connected to pins 1 and 2 internally. But, since you must use a 7189A, the 300Ω resistor between pin 6 and pin 9 is useless because pins 6 and 9 are also internally connected, thus shorting the resistor. This is not logical.

Is the drawing correct?

BTW, I ran into the weird 7189A pin connections on my Magnatone M10A. I wanted to put a pair of EL84s in the amp, but my 7189A sockets had the screen connection at pin 6 on the socket. Won't work unless I move the wire to pin 9 or put an external jumper between pins 6 and 9.
 

[kagliostro]

Better I post a photo

may be the answer is here ?

On my GE 6BQ5 tubes which would be assumed to correspond to their own handbook, everything is as I listed above, and also pins 1 & 2 are jumped internally. Pins 6 & 8 are there going through the glass, but have no electrical connection to each other or anything else inside. These GE tubes have 6BQ5 USA etched on the glass with the typical dot pattern below, in white writing there was added GE 6BQ5/EL84. The box also says 6BQ5/EL84.

Kagliostro

[sluckey]

Sure can't argue with that.

[kagliostro]

Sure can't argue with that.

that wasn't the reason for what I posted the photo 

I still do not really understand if the connections on the socket were to fit the 7189A or not 

It is late and I'm tired

Kagliostro

[sluckey]

I still do not really understand if the connections on the socket were to fit the 7189A or not 

I don't think so. And I base that on the fact that the 7189A has pins 6 and 9 tied together indide the tube. Therefore, the resistor mounted between pins 6 and 9 is useless. My Maggie has 7189As and I know that the screen connection is made via pin 6. I use pin 9 for screens on my EL84 amps. Also, all the 7189As I see in various tube manuals always shows pins 6 and 9 connected together internally. Same for pins 1 and 2.

There does seem to be a bit of confusion with the 6BQ5, EL84, and 7189 diagrams though.   

[PRR]

You can use modern "EL84s" at the high voltage often used on 7189 and get the same output.
 

[kagliostro]

Thanks Sluckey & PRR

Sluckey

Ok for the explanation about pin 6 tied internally with pin 9, yesterday I was very .........

PRR

in your experience which is the max voltage that can be apply to modern el84 ?

with a higher voltage the standard 4Kohm OT is fine or must be changed the Plate to Plate impedance ?

I'm not looking for more than 30W x 4 tubes, but with 4 modern el84 in which rate can be the max power ?

Thanks

Kagliostro

[PRR]

> the max voltage that can be apply to modern el84

I had no trouble with Hoffman's (Sovtek?) EL84 at 410V.

Is this a new from-scratch amp or an old amp?

If old: add a tag-strip so grid-stop resistors do not have to connect to "unused" pins which may not be un-used on different versions of EL84/7189, then use EL84.

Power was rated 17W/pair EL84 at 300V self-bias, 24W/pair for 7189 at 400V fixed-bias.

Since this is a self-bias amp, there was little reason to go over 300V; however 348V may cover bias voltage and full-power sag and is fine.

[kagliostro]

Is this a new from-scratch amp or an old amp?

If old: add a tag-strip so grid-stop resistors do not have to connect to "unused" pins which may not be un-used on different versions of EL84/7189, then use EL84.

I agree with you about to prefer to have no components connected to the "unused" pins, good council

the amp is old and I don't like how is wired (PTP & Tag Strips, but in a bad way, also with many ground connections at Tag Strips), so I think to rebuild with a Turret board

The original PS has SS rectify  and with 222v line (that I measure at my home) connected to the 220v input of the PT I've 366v-0-366v & 7v AC

366v x 1.4 = near 510v that seems too much to me also if there are two big resistors (150ohm 15W) between PT and the SS rectifier (see the schematic posted previously)

if I connect the 222v line to the 240v input of the PT I've 337v-0-337v & 6.4v AC

337v x 1.4 = near 470v that to me seems also high for the el84 tubes

Since this is a self-bias amp, there was little reason to go over 300V; however 348V may cover bias voltage and full-power sag and is fine.

You are right, so I don't understand that PT, it seems more reliable for other tubes 

Many thanks

Kagliostro

[PRR]

> there are two big resistors (150ohm 15W) between PT and the SS rectifier

Yes; and if I simulate a 48 Watts (four 12W tubes) 400V 120mA or 3.3K load, the huge drop in these resistors gives about 400V.

I would assume the Designers knew what they were doing (by calculation or by test).

I would guess they did not find a stock PT with the right voltage.

Yes, before the EL84s warm-up there will be a LOT of voltage.

[tubeswell]

The 6P14P-EB seems to be able to take higher voltages and last

[kagliostro]

Yes, before the EL84s warm-up there will be a LOT of voltage.

it would take something like the old brimistor

but which could be a modern replacement ?

The 6P14P-EB seems to be able to take higher voltages and last

6П14ПEВ (I think you mean 6P14P-EV)

Interesting tube

Thanks

Kagliostro