UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 14th Nov 2015, 2:24 pm   #41
SurreyNick
Heptode
 
Join Date: Apr 2014
Location: Wigton, Cumbria, UK.
Posts: 728
Default Re: Simple battery-powered valve guitar amp

OK David. You're right. Time to add some specifics.

It's a 20 watt solid state 'busking' amp that I have designed. My son is into heavy metal in a big way and so the circuit includes an MXR Distortion Plus as the preamp and a TDA2005 12V 15/20W amp, driving twin Jensen MOD5-30 speakers. The amp is powered by a Ritar RT1250 12V 5Ah SLA battery and I have included a circuit to charge the battery from a 12v wal-wart with automatic cut-off and from which the amp can also be powered.

I have added to this post a picture of the cabinet design, which has two wings to hold the 5" speakers, and measures just 22cm x 22cm x 28cm. The amp will weigh just under 5kg with one battery, but the battery compartment is large enough to hold two, so fully loaded it will weigh just over 6.5kg.

For a valve amp comparison I'd like to maintain the 20 Watt output and keep to the same dimensions for the cabinet. I'd also like to still drive twin Jensen MOD5-30 speakers. My son is very muscular so he won't have any problem lugging around anything under 10kg.

Nick
Attached Thumbnails
Click image for larger version

Name:	Basic Cabinet Design (small).jpg
Views:	138
Size:	15.0 KB
ID:	115565  
SurreyNick is offline  
Old 14th Nov 2015, 3:02 pm   #42
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Simple battery-powered valve guitar amp

20W puts him into EL34 territory, or maybe 4xEL84.

Instead of having a 12v battery and upping the ampere hours, why not go for a higher voltage? 4 SLABs for 48v. Your inverter efficiency will be better.

Series string heaters and TV line output valves could dodge the hyped valve types.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 14th Nov 2015, 3:21 pm   #43
turretslug
Dekatron
 
turretslug's Avatar
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: Simple battery-powered valve guitar amp

1625 "toobs" ought to be tough as old boots, cheap and would keep heater current down as part of a 48V series string.
turretslug is offline  
Old 14th Nov 2015, 4:08 pm   #44
SurreyNick
Heptode
 
Join Date: Apr 2014
Location: Wigton, Cumbria, UK.
Posts: 728
Default Re: Simple battery-powered valve guitar amp

The EL84s are a nice choice because they are small, and physical size is a factor because I want to keep the cabinet at 8.5" x 8.5" x 11". Even better would be those small Russian rod valves. I have a whole heap of those and nothing planned for them yet.

I have about twenty 1J29B-V HF Pentodes, a similar number of 1J18B HF Pentodes, and a dozen or so 6N16B-V double diodes.

I know so little about valve technology I don't know how much these little things can be pushed. To date my experience is limited to TRF receivers and solid state practice amps. Moving to a battery-powered valve amp is a big step in my knowledge curve, but I really do want to give it a go.

Note: I was planning to do as Juile suggested too, i.e. a blind test (solid-state vs valve). Make two identical looking amps and simply hand them to my son and say "Give them a go, tell me what you think of their performance, and can you guess which is valve driven?"

Nick

Last edited by SurreyNick; 14th Nov 2015 at 4:14 pm. Reason: Addition
SurreyNick is offline  
Old 14th Nov 2015, 6:33 pm   #45
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Simple battery-powered valve guitar amp

Nick, before you shell out any cash for the valve amp project it would be worth while finalizing the circuit of choice together with approximate efficiency figure, eg power in (batteries) to power out (to speaker) expected current draw time from charged to discharged etc

Are the batteries or battery to be external to the cabinet or internal?

Lawrence.
ms660 is offline  
Old 14th Nov 2015, 7:23 pm   #46
SurreyNick
Heptode
 
Join Date: Apr 2014
Location: Wigton, Cumbria, UK.
Posts: 728
Default Re: Simple battery-powered valve guitar amp

Very good advice Lawrence. I have, in the past, rushed ahead and bought items in the belief they were what I wanted/needed and then realised I'd wasted my money. That's how I ended up with the mini Russian rod valves and also some 1L4 and 1F2 valves.

The batteries are internal. If you refer to the picture in post #41 the speakers go into the wings, the electronics in the bottom compartment and the batteries in the back compartment. The back compartment is wide and deep enough to accommodate two Ritar RT1250's side by side.

I have just finished reading through all the posts in this thread and have come to the conclusion I'm going to need a lot of help and hand-holding to come up with a circuit for this valve version. Consequently I'm wondering if I should perhaps abandon it until my knowledge and experience is better

I always want to run before I can walk.

Nick
SurreyNick is offline  
Old 14th Nov 2015, 8:18 pm   #47
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Simple battery-powered valve guitar amp

According to the manufacturers data the 5 Ah capacity is at the 20 hr rate down to a cell voltage of 1.75 volts, I'm not to well up on battery characteristics but I think it means a load drawing 250 ma for 20 hrs, I am led to believe that hours versus load current draw based on the quoted Ah capacity is not a linear one, someone else might be able to tell you more on that than I can.

A pair of EL84's in a series heater string would draw over 750 ma on top of that there's the current draw of an inverter/converter to deliver the required HT current for the amplifier.

You could have the two batteries in series but you might find some power wastage when trying to configure all the valve heaters if shunt resistors are needed, on the other hand a 24 volt converter for the HT might run more efficient than a 12 volt one, I usually find it's all swings, roundabouts and trade offs with this sort of thing.

Lawrence.
ms660 is offline  
Old 14th Nov 2015, 9:38 pm   #48
Guest
Guest
 
Posts: n/a
Default Re: Simple battery-powered valve guitar amp

As a pulled from the hat guess, running a 25W valve amp for 5 hours would need something about 1/2 the weight of a car battery if using lead acid. Lithium Ion would weigh a lot less (model aircraft batteries) but have a more varied discharge characteristic.
 
Old 14th Nov 2015, 9:49 pm   #49
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Simple battery-powered valve guitar amp

The heaters are going to take a big bite of the total power, especially as they run full-whack all the time. HT power will get a bit of a rest between power-chords and between numbers.

As the overdrive characteristics of the output bottles are going to dominate the sound effects, then does the rest of the amplifier need to be valved? Drivers and preamps don't use much HT power, but their heaters are significant. Maybe a compromise would suit?

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 14th Nov 2015, 11:02 pm   #50
SurreyNick
Heptode
 
Join Date: Apr 2014
Location: Wigton, Cumbria, UK.
Posts: 728
Default Re: Simple battery-powered valve guitar amp

Perhaps a compromise would suit David yes.
I'll have to look at those Lithium Ion batteries too.

To start I need to lay my hands on a schematic which will work, or which can be adapted to suit. Hi-Ho, Hi-Ho, a hunting I will go....

Nick
SurreyNick is offline  
Old 14th Nov 2015, 11:49 pm   #51
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Simple battery-powered valve guitar amp

For a guitar amplifier, you don't want negative feedback around the output stage. That would act to reduce the 'valvishness'. On the other hand, if the preamps and drivers are solid state, then you do want feedback around those stages.

I'd suggest using an interstage transformer as the phase splitter, and a bit of step-up ratio would help get the required g1 swing for the finals without requiring high voltage transistors as drivers.

The data sheet for the finals will give the anode-to anode load impedance the output transformer must give, so that sets the output transformer ratio. The low frequency 3dB point you want sets the transformer inductance... so one can be wound from that data.
Datasheet grid bias voltage and quiescent current give the cathode resistor values. Cathode decouplers come from that resistance and the LF 3dB point.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Nov 2015, 1:20 am   #52
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Simple battery-powered valve guitar amp

EL84/6BQ5's are easy to drive, requiring only 10 volts per bottle.
HT draw is 2 times 57 mA at full tilt, 40 mA zero signal
Heaters are 6.3 volts @ 760 mA
Load P-P is 8K so transformers are easy.

Transistor drivers would be most economical as far as current draw goes.
Valve preamplification would definately sound better but will either reduce the playing time, or require larger batteries for equivalent times.

You will still require an inverter of some sort to get the HT!! 300 volts in the above example, I will leave the design of that to somebody more familiar than I. I havent done much inverter design lately, but there are some modern mosfets that will do the job nicely.

Assume we only have the two EL84,s inside the box, you will need some reasonably efficient way of removing the heat, or you will be frying the battery in its own juices.

I have made up a few systems like this, but I cheat!! A small 12 volt to 240 volt inverter ( sine wave of course) can be had cheaply but they still require large battery power and I mount the inverter and battery in a carry box, and use a conventional guitar amp.

Just my musings on the thread as I still do a fair amount of guitar amp work,

Joe
joebog1 is offline  
Old 15th Nov 2015, 8:40 am   #53
Guest
Guest
 
Posts: n/a
Default Re: Simple battery-powered valve guitar amp

How about a low power valve amp (battery?) then a transistor output.
 
Old 16th Nov 2015, 2:54 pm   #54
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Simple battery-powered valve guitar amp

6V6s or 6BW6s will deliver similar power to EL84s with less heater current. Four in p-p parallel could run off 24V battery with all heaters in series. I guess a suitable high voltage mosfet would do for a concertina type phase splitter just used like a valve for similar duty. Solid state front end will be happy on 24V too with suitable devices or just an opamp.

With a stab in the air guess at overall efficiency around 20%, 20W for 5 hours will need about 500WH. At 24V that's about 20AH. That's going to to need at least a couple of 12V 30AH or so batteries- two small car batteries size wise for SLA. If the valves are biassed back nearer to class B and the drive power upped this could be improved somewhat. The extra distortion might actually be desirable
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 16th Nov 2015, 2:59 pm   #55
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Simple battery-powered valve guitar amp

Quote:
Originally Posted by merlinmaxwell View Post
How about a low power valve amp (battery?) then a transistor output.
The one in the youtube video followed up by a cheap class D solid state booster should do nicely for that.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 16th Nov 2015, 3:39 pm   #56
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Simple battery-powered valve guitar amp

Fascinating!
Would a 1.5V battery valve like the DL96 pentode and/or the DAF96 (audio triode/diode) be any good for preamplification stages? Maybe wire the DL96 as a triode and use as a concertina Phase splitter. At 50mA heater draw it wouldnt be pulling much out?

Just curious, I have often wondered how those little valves would work in a quality preamp.

A.
bikerhifinut is offline  
Old 17th Nov 2015, 6:18 pm   #57
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 665
Default Re: Simple battery-powered valve guitar amp

I like the idea of a low power valve stage, overloaded in true guitar amp fashion, followed by a (not overloaded) transistor "linear" amp.

Using battery valves, possibly even the Russian rod valves, you could greatly reduce the inverter requirements and hence load on the battery, and a transistor "bridge" amp as used in car radios should give 20W without overloading from 12V.

With a volume control after the overloaded stage, you should even be able to get the right overload effect at less than full volume. If required.

Does a transistor amp using an output transformer have a similar overload response to a valve amp, or did no one ever build a transistor amp with a small enough transformer?

Stuart
stuarth is offline  
Old 19th Nov 2015, 9:07 pm   #58
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Simple battery-powered valve guitar amp

I was leafing through my Mullard Equivalents and valve data book looking for some info on the EF86 types and I found the entry for the EF98.
B7G
Vh 6.3V Ih 300mA.
Va 12.6V Ia 4.8mA
Ig2 2.2mA.
ra 50k ohm.

looks like a gain of 8 (amplification factor vin/vout) in tetrode mode so theres a bit of distortion from the terode kink?

Finally its described as a "Pentode for use as a driver stage in car radios having a transistor output stage"
Maybe useful for this project?
Of course I have no idea how available they are

A.
bikerhifinut is offline  
Old 20th Nov 2015, 12:51 am   #59
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Simple battery-powered valve guitar amp

How about those lithium ion battery packs for power drills. 4 amp-hours at 18v in a small package? It's a multiple of 6v so not too bad for heaters and high enough for reasonable efficiency in an inverter. chargers are also available for them. Not cheap, but neat.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 20th Nov 2015, 11:57 am   #60
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: Simple battery-powered valve guitar amp

I suggested using drill batteries in post 4 David, they pack a lot of power into a small package. The is a video on Utube of one being used to start an artic lorry, Gerry Sweeney's channel I think.

Whether a battery designed for high current output is better in this application than one designed for low current, long life I have no idea.

With valves it's the heater current you have to watch. I built the Valvecaster distortion pedal that runs on 9v or 18v, it will run on a PP3 but not for long. I wonder if you can underun the heaters on the valves on the pre stage and use transistors on the OP stage but site the trannys next to the valves so the heat dissipated improves the electron flow from the cathodes to make up for the underun htrs. Or could you run a valve without the heater but use it as a heatsink for a tranny OP stage. Just a whacky idea, but it seems you are going to have to build some clever power saving ideas into your amp.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:57 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.