• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

$120 battery powered tube amp on eBay??

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#23
2 "D" Cells?

How pray tell does the two 'D' cells source enough current to drive the filiments?
Fran:
Car amps use SMPS' using just 12 VDC to crank up the Vcc to usefull levels.
They go thru another step in the supply process to get to Vcc.
The key in car amps is that the 12 VDC has virtually unlimited (1st Appx) current available.
________________________________________Rick............
 
#27
3 V is more than enough

The 1s4 is a good but cheap direct heated tube that was designed for portable radio operation. It is an amazingly good tube that is good for a headphone amp. If you have halfway efficient headphones one tube per channel should not be any problem. If you want to drive an eight ohem load you can parallel two tubes and adjust the circuit values accordingly.
I assume that the circuit has a basic power inverter to jump the 3V to 45V that is needed for the tubes. That really is all they need and is the sweet spot for these tubes. You can get 45V batteris easilt and these do last a long time in a headphone amp. You can also of course wire 5 x 9V batteries in series which is neither rocket science nor expensive. I would assume several hundred milliwatts of power at this voltage and power is conserved so 5 nine volt batteries will give you a lot of listening time. The fillaments are 1.5 volts and do not draw much current either.

Given that this tube is a very old American design and that it was used in old radios back in the day it is hard to imagine that any other knockoffs other than American made exist. I think that these are very good sounding tubes and they will really beat 90% of what is out there under extraordinary prices. There have been several articles over the last 15 years or so on the audio merits of these tubes in headphone amps and finding these may be worth your time in a library. I highly suggest these tubes to anyone and given their ease of use and cost you really cannot go wrong.

I would personally make my own amp without a kit especially considering how simple single stage class A amplifers are. The RCA radiotron designer's handbook can be your best friend if you want it to be... and you do not have to steal a copyrighted design to make a decent amp.

Mike
 
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#28
Well it took about an hour this morning for my 7-year old to put one of these kits together. The look on his face when he turned it on and heard his own music from his tubies was priceless.

I don’t have the heart to take it apart yet and provide voltage measurements but I will do that. There is a chopper step-up of some sort but I don’t know what B+ voltages are. The speakers are not bad considering, and provide more then enough volume for his satisfaction, and no bass output for my satisfaction. But I will hook the amp up to my La Scalas anyway and see what it sounds like.

The kit is self contained with everything needed for assembly except 2 “D” cell batteries. The only tool needed for assembly is a small Philips head screwdriver, one is included in the kit. We were both a little disappointed there was no soldering involved, but then I was feeling slightly nervous about introducing him to lead and flux fumes anyway.

He is so excited about completing his first tubie project, which is what it is all about.
Matt
 
#30
Hi

It's quite a nice toy, for the kid in the man 😀

The circuit:

An externally hosted image should be here but it was not working when we last tested it.


My implementation:

An externally hosted image should be here but it was not working when we last tested it.


Using it with high efficiency speakers turns out to be a nice little amp:

An externally hosted image should be here but it was not working when we last tested it.


For me, it is like playing with a model train: having a model amp (having replaced the ceramic caps in the signal path by common MKP types).

Have fun
Franz
 
#32
Do you guys think that this setup can be used as a computer amp/speaker setup?
Click to expand...

Yes, why not?

But not with the horn speakers, you will not get a satisfying sound reproduction.

You need some high efficency speakers, about 96dB/W.

And dont forget: under 100Hz and above 10kHz there is nothing, very limited bandwidth.

And you need a decent power supply, 1,5V DC at 350 to 400mA.

This is not a wallplug job! You really have to use a 5VAC tranny with sufficient VA (more than 10VA) and very good filtering / regulation to get clean 1.5V.

I did not get it, to make a PSU without hum. I used 22000uF cap, LM338 regulator, and another 22000uF.

And the thing is still humming.

This tubes are designed for battery operation. And the PCB is not optimized to use with a PSU: the ground plane forms a big loop around the whole circuit.

Kind regards
Franz
 
#34
Hi there.

Couple of questions about the Gakken amp:

Has anyone mangaed to sort out a non-humming main power-supply for this amp?

Has anyone tried hooking it up to a set of proper hi-fi speakers?

I'm currently using a non-modded T-amp with a set of of horn speakers. I need hardly any power at all and the idea of running the horns with a cheapo tube amp is tempting.

Cheers,

Mike.
 
#35
I’ve been meaning to post back with voltages, and now that this thread is active again here is what I get for voltages using the schematic posted above, which comes with the kit,

Voltages, Batteries are reading 1.33V when turned on,
1B2 Plate to ground , #1 -0.002V, #2 N/A, #3 (R) 1.91V (L) 1.47V, #4 (R) 17.11V (L) 16.6V, #5 (R) 18.4V (L) 16.6, #6 (R) -1.4V (L) -0.05, #7 (R) 1.25V (L) 1.25V,

2P3 Plate to ground #1 1.25V, #2 (R) 38.9V (L) 40.2V, #3 (R) 41.5V (L) 41.4V, #4 (R) -3.6V (L) 4.7V , #5 N/A , #6 (R) 40.12V (L) 40V , #7 (R) 1.24V (L) 0.05V,

After doing this I see I have some tube matching problems which I will have to attend to at some point. While I cannot say anything specific on battery life, they last longer then I expected. It seems like my boy can get three or four weeks out of a pair of “D” cells doing about 10 hrs of listening a week.

I have hooked this up to my La Scalas and while I had a little hum, nothing was objectionable. However it sounds much better driving Fostex FF85Ks then the horns, though my son prefers the horns since he made them. Where have I seen that builder’s blindness before?
Matt
 
#36
Just got one of these. Fun little toy! More volume than I expected for 100 mW/channel. Just using it as sound for my computer right now. Using some old Realistic 8" full range speakers. Sounds nice, but lacking in the bass dept, and distortion prone if you try to EQ it to compensate. Don't have the money to blow on high-efficiency speakers, but I might try some NOS 8" Marsland public address speakers that are allegedly 98dB sensitivity. (and dirt cheap!)
 
#38
Puguglybonehead said:
An externally hosted image should be here but it was not working when we last tested it.


Was looking at the schematic. Is there any hope of improving the bass in this thing? I was thinking of swapping out R1 for something larger.
Click to expand...

I wouldn't if I were you, this stage is probably using grid leak bias (R1) and changing that resistor will affect the operating point of the stage. You could try changing C1 to maybe 0.015uF or so, note however that this could result in output transformer saturation at low frequencies.
 
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