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Old 11th November 2013, 05:08 AM   #1
w5jag is offline w5jag  United States
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Default A Simple Child Resistant SSE

It's been about two years since I made any contribution to the Tubelab forum. For some time I've wanted to make a low power 6V6 SSE. The birth of our third child (almost three decades since the last ), and the arrival of his toddling and exploratory time, made a child resistant SSE mandatory. The want and the need converged, and the result is a child resistant, low power, SSE, shown here.

Kid protection is done the old fashioned way - an old school, perforated steel cage. These are an off the shelf item available from Hammond, in several sizes and finishes, including 12" x 10", a chassis size I already had on hand, mostly cut and drilled, the remnant of a 45 / 2A3 Tubelab SE, now dismantled. This made construction fast and easy. A few unneeded holes were filled on the donor chassis, the power transformer, a flat mount Utah of unknown vintage, was reinstalled. It is well suited for a low power SSE; it is heavy, loaded B+ output is 320 ish with a CLC circuit, and 275 ish with an LC circuit. It does not have screen voltage secondaries. Output transformers are the small Edcor XSE-15-8K, replacing the Transcendars that were used with the Tubelab SE.

A new SSE board was assembled almost in the usual manner; the dropping resistors in the CCS circuit were omitted because of the low B+ voltage. All components are top mounted for quick access for future modifications.The board is installed on top of the chassis, under the cage.

To keep heat down, initially I elected to use solid state rectification, and put a couple of silicon rectifier diodes on the board. An additional benefit of this was that the 5 volt secondary became available for other uses, including a low voltage supply for possible future solid state circuitry, or for a screen grid supply voltage. The former can be achieved by rectifying the output and running it into a voltage doubler or tripler for a solid state circuit, and the latter by combination with a second, small, filament transformer, thus:

120 : 5 = 5 : 120

and then rectifying and filtering the 120 volt AC output in the usual manner for screen voltage. Having done this previously in other projects, this yields output in the 170 - 180 volt range, suitable for small tubes like 2E26, 6BQ6, 6W6 and 6Y6 that require a low screen voltage when used as pentodes.

Later, I did decide to install a tube rectifier. To keep the 5 volt secondary free, I chose a 6AX5GT. This was selected as it can be fitted on the SSE board with no destructive modifications, needing only a few jumpers on the socket traces to rearrange the pins, and 6 volt filament power ran to pins 2 and 7 on the socket. This tube has low noise operation, and power output comparable to the 5Y3. The 6X5 can also be used without further modification, however, it has only about 65% of the power output of the 6AX5. Both of these tubes are plentiful and inexpensive.

Another oddball feature is the 3.5mm stereo phone jack on the front panel as a secondary audio input. This is wired in parallel with the rear panel inputs, a dubious practice from a technical standpoint, but one that works well in actual practice, and is quite convenient.

Presently, the power supply uses a 4.7uF first capacitor and a 10H inductor. B+ is 320 ish, depending on the power tubes. The cathode resistors are 680 Ohms. THe feedback at the output tubes is ultralinear. Cathode feedback is not (yet) enabled. This puts 6F6 at about 11 watts, 6V6 at about 10 watts, and 6K6 at about 11 watts (6K6 is an 8 watt tube). I have not tried 6G6 yet, or 6W6 and 6Y6, as the latter two require a low screen voltage and pentode wiring, or triode wiring. 6L6, 6L6GA, 6L6GB, and 5881 could also be used.

A few pictures are attached. It is rough looking, being hastily thrown together out of materials on hand, the cage being the only part I had to acquire. The large, off board, filter cap is not presently connected. It fills a hole where a meter resided in the Tubelab SE. I had intended to use it, but the amp is stone cold silent on 96db ish speakers without it, so it remains unconnected. Paint is a possibility, if the demands on my time relent at some point.

This is a nice sounding amp; the 6V6 is one of the finest sounding audio tubes, imho. It has been in service a couple of months without issue.

Win W5JAG
Attached Images
File Type: jpg IMG_20130923_235744.jpg (836.0 KB, 1256 views)
File Type: jpg IMG_20130924_000555.jpg (952.6 KB, 1204 views)
File Type: jpg IMG_20130929_223930.jpg (839.1 KB, 1152 views)
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Old 10th December 2013, 10:55 AM   #2
BillEpstein is offline BillEpstein  United States
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I bought and installed the Hammond cage on my tube rectified, KT-88 powered SSE and within a few minutes, it was too hot to touch.

The cage is now in the closet and I just don't play it when the granddaughter is around.
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Old 10th December 2013, 02:35 PM   #3
w5jag is offline w5jag  United States
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As you note, if it's too hot to touch, it's not suitable to be around kids.

Two KT-88's, plus a 5AR4, and a 12AT7 - probably a couple of hundred watts in heat right there, before you even get to the heat generated by the power transformer. If it's undersized, it just makes things worse. An exhaust fan might help you, but it still may be too much heat, in too small a space, even with a fan.

That's why mine is sized the way it is. Two twelve watt tubes, a 12AT7, and a 6AX5, maybe sixty five watts total heat from the tubes, and quite a bit of empty space between the top of the tubes and the cage. The power transformer is oversized, so it runs cool to the touch, and it's a flat mount - much of the minimal heat it generates is sinked to the chassis and spread around, rather than cooking a spot on top of it.

My cage runs at about room temp, even after many, many, hours of use. Cool enough that I could probably go up to the 19 watt class of tubes, like 6L6 and 5881. That's probably about the limit of the 6AX5 rectifier, although I have diodes on the board, if need be.

When I was first pondering this, I considered using an LMB Heeger CO-6 ( 9 x 11 x 6 ) cabinet. The advantage of this was that I had one on the shelf*. The disadvantage was that heat management looked dicey, at best, and I would have to use an upright power transformer, or an undersized flat mount, and the output transformers would have to go on the back of the cabinet, under a sub cabinet. It would have been do-able, probably, with a small exhaust fan, but for long term reliability and simplicity, I just went with the cage. It's working out very well.

If I were going to do it with a "big" amp, I would start with the biggest chassis and cage Hammond makes ( 17 x 10 ?) and make provisions in advance for an exhaust fan, in the event chassis size alone was not sufficient to dissipate the heat.

Win W5JAG

* I use a lot of the LMB Heeger cabinets - they are a decent looking off the shelf product. I have a scratch built two meter rig in a C0-4, my original SSE was built on a C0-1 chassis that originally was used for a scratch built 75 meter ssb transceiver ( my second big project ). Ironically, while rummaging around in the warehouse of junk a few weeks ago, I found the C0-1 cabinet for the chassis, so the old warhorse original SSE may yet ride again, in a fully enclosed C0-1 cabinet. Or maybe the Tubelab SE will be resurrected and go in that cabinet as a child resistant TSE.
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Old 19th January 2014, 12:10 AM   #4
shortwave is offline shortwave
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I built a EL34 based SSE a couple years ago. After a couple of very busy years I'm getting some free time again. I'm just a novice at this so thanks so much for sharing your ideas and project.
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Old 20th March 2015, 01:26 AM   #5
w5jag is offline w5jag  United States
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Default alternate tube data

Tubelab provides model data for the 6V6 in the SSE. For good reason, most low power variants of the SSE likely use 6V6. Here is some data on alternate tubes for use in low power SSE amps, specifically the above described child resistant SSE.

6Y6: Ordinarily, I would think this tube unsuitable at the voltages present in my child resistant SSE. It specs at low plate voltage, and even lower screen voltage. However, an application note in an old Acrosound Transformer catalog describes superlative performance of this type in a push pull ultralinear configuration, at voltages similar to my amp. 6Y6 has a large heater current requirement - 1.25 Amps, approximately three times that of the 6V6.

The 6Y6 I had on hand are JAN GE 6Y6GT, 1966 vintage, previously unused. The spec data given below is from the GE data sheets. I have seen different specs. but went with the GE data, since my tubes are GE.

6Y6 in child resistant SSE, measured

Ultra Linear configuration

anode 287 spec 200
screen 290 spec 135
cathode 41.5

Rk = 680
(41.5/680)=0.061 (246)(0.061)= 15 watts spec 12.5 watt

This is serious spec busting (this is the Tubelab forum, after all) and a cathode resistor of 820 ohms, perhaps larger, would probably bring this tube into spec. That said, they don't show any distress at 15 watts, no sign of color on the plates, or other issues at the elevated voltages found in my amp.

6W6: 6W6 also has a large heater current requirement - 1.25 Amps, approximately three times that of the 6V6.

The 6W6 I had on hand are 70's vintage, coin base GE 6W6GT, previously unused. The spec data given below is from the GE data sheets. I have seen different specs. but went with the GE data, since my tubes are GE.

6W6 in child resistant SSE

Ultra Linear configuration

anode 306 spec 300
screen 309 spec 150
cathode 33

Rk = 680
(33/680)=0.049 (273)(0.049)= 13.375 watts spec 10 watts

This is more spec busting, and a cathode resistor of 820 ohms, perhaps larger, would probably bring this tube into spec. They do not show any distress at 13 watts, no sign of color on the plates, or other issues at the elevated voltages found in my amp.

I would not consider using these in any configuration than ultra linear at these voltages. The screens have too low a rating for pentode or triode at these voltages. I have used both as triodes at 210 or so volts with out any issues and with good results.

Russian 6F6S: These are sold on Ebay as an analog to the American 6F6 and mine measure remarkably similar. I believe they can be considered as 6F6 for all intents and purposes. Mine are the small shoulder type, early 60's vintage. I am not entirely sure on the specs; I don't read cyrillic.

anode 333 spec 410
screen 334 spec 315
cathode 25

Rk = 680

(25/680) = .037 (308)(.037) = 11.4 watts spec 12 watts

These tubes sound good. They are somewhat expensive compared to american 6F6.

Russian 6L6GT: aka 6P3S, These are sold on Ebay as 6L6GT, an analog to the American 6L6, and they are a reasonable analog to the original 19 watt 6L6. I believe they can be considered as 6L6 or 6L6GA (NOT 6L6GB or GC) for all intents and purposes. Mine are Reflector made, unknown vintage. I am not entirely sure on the specs; I don't read cyrillic.

anode 314 spec 400?
screen 317 spec 300?
cathode 33

Rk = 680

(33/680) = .0485 (281)(.0485) = 13.6 watts spec 20.5 watts?

I have written bad things about these tubes before; I take it all back. I think they sound terrific. I have put probably over a thousand hours on mine, they show no degradation, sound terrific, and are a great value.

One other note: my GE 6AX5GT rectifier shows visible signs of taking a beating, but does not show any evidence of electrical degradation thus far. The type I use has the innards of a 6087/5Y3WGTB.

Two other small types might work: 6U6 and 6M6. I have some 6U6 in my warehouse, but had none here at the house to test. I have never seen 6M6. I question whether it actually exists, or was used here in the U.S.

Hopefully, this data will help other builders of low power SSE.

Win W5JAG

Last edited by w5jag; 20th March 2015 at 01:45 AM.
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Old 20th March 2015, 01:48 PM   #6
spendorite is offline spendorite  Canada
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Thanks for the info W5JAG. I have a second SSE for use with 6V6, 6K6 and 6F6 tubes. B+ with a 5AR4 rectifier is 340V and
with a 5V4 rectifier is around 320V which is what I use mostly with these tubes.
I have some RCA and Sylvania 6Y6G tubes. Using a GE 6087 rectifier I get a B+ of around 290V so I tried these tubes in there
with 900 ohm cathode resistors. After many hours of use I see or hear no indications of damage to the 6Y6 tubes.
From memory the measurements I took were about 5% above spec.
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Old 20th March 2015, 02:28 PM   #7
w5jag is offline w5jag  United States
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What configuration are you running? Triode? That is useful information to know, so thank you for posting.

Perhaps other builders will post and we can start compiling some data on these smaller tubes in the SSE.

Win W5JAG
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Old 20th March 2015, 04:23 PM   #8
spendorite is offline spendorite  Canada
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I tried triode config for a short while but switched to UL because this sounded better with my speakers
which are only rated at 89db efficiency but presents a very tube friendly load.
I've been using UL mode ever since. I really have to get a pair of good high efficiency speakers.
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Old 22nd March 2015, 10:13 PM   #9
w5jag is offline w5jag  United States
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Default Some additional data

6BQ6GTB in child resistant SSE

6BQ6 also has a large heater current requirement - 1.25 Amps, approximately three times that of the 6V6. This is a pair of Tung Sol tubes, data is from a GE data sheet, my Tung Sol books are boxed from a move.

Ultra Linear configuration

anode 282 spec 600
screen 286 spec 200
cathode 40

Rk = 680

(40/680)=0.059 (242)(0.059)= 14.25 watts spec 11 watts

It's easy to see where the 6AX5 rectifier runs out of gas and experiences a pretty significant voltage sag beyond its spec - excluding the current drawn by the 12AT7, it's somewhere just past 49 ma per power tube, or in excess of about 13.5 watts. Assuming 20 ma or so for the 12AT7, that looks to be right in line with the data sheets.
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Old 23rd March 2015, 06:31 AM   #10
w5jag is offline w5jag  United States
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Default 2E26 and 6146 in the Child Resistant SSE

Transmitting tubes 2E26 and its higher powered follow on, 6146, are easily adapted to the SSE board. Details can be found in an earlier thread in this subforum; briefly plate voltage must be sent to the anode cap, screen voltage must be applied to pin 3 of the power tubes, and pin 4 of the power tubes must be left open as it is internally connected to the cathode in these types. Because of the unique configuration of the SSE board, making these changes is trivial.

2E26 in the child resistant SSE:

The 2E26 has an 800 ma heater current. This is almost double the current drain of the 6V6. Data is taken from the 1947 RCA data sheet.

Ultra linear configuration:

Anode 320 spec 300
Screen 323 spec 200
Cathode 28

Rk = 680 ohms.

(28/680) = .041 (.041)(292) = 12 watts. The specification for continuous duty (CCS) service is 10 watts maximum.

I continue to be impressed at how adaptable the 2E26 is to audio amplifier service. They always seem stable, tolerant of reasonable out of spec operation, and just sound good.

6146B in the child resistant SSE:

The 6146 has an 1250 ma heater current. This is almost triple the current drain of the 6V6. Data is taken from the 1964 RCA data sheet.

Ultra linear configuration

Anode 275 spec 600 (CCS)
Screen 279 spec 250 (CCS)
Cathode 44

Rk = 680

(44/680) = .064 (231)(.064) = 14.75 watts. The plate dissipation spec for continuous duty (CCS) is twenty watts. It is well within all specifications. It works well and sounds very good in this configuration. RCA deemed it a compact powerhouse and it is an apt description.

Several things are apparent: First, the high perveance characteristic of 6146 - it's happily sucking down almost 65 milliamps cathode current at a mere 231 volts across the tube. It's a sweep tube and needs to be treated as such.

Second, the extreme voltage sag of the 6AX5 rectifier - when it runs past its current spec, the output voltage just falls off a cliff.

Third, 2E26 is typically portrayed as just a low power 6146, but it really doesn't behave much like the 6146. In all configurations, 2E26 behaves more like an ordinary audio tube. It has a low screen voltage rating, but it doesn't act much like a sweep tube, whereas 6146 is very much a sweep tube.

Win W5JAG
Attached Images
File Type: jpg 6146 CRSSE 2.jpg (865.6 KB, 741 views)
File Type: jpg 6146 CRSSE 3.jpg (953.1 KB, 692 views)

Last edited by w5jag; 23rd March 2015 at 06:43 AM. Reason: added pics
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